feat(pocket-base): add purchasable biome changes

src/main/java/games/dmg/spigotbase/BaseSettingsCommand.java

@@ -20,6 +20,9 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
     );
     private static final List<String> VISITOR_MODES = List.of("allowed", "blocked");
     private static final List<String> ENABLE_MODES = List.of("enable", "disable");
+    private static final List<String> POCKET_WORLD_TYPES = List.of(
+        "void", "nether", "overworld"
+    );
     private final BaseStateManager stateManager;
     private final PluginSettingsProvider settings;
     private final TeleportPolicy teleportPolicy;
@@ -70,6 +73,10 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
                 && arguments[1].equalsIgnoreCase("mobs")) {
             return updatePocketMobSpawning(player, arguments[2]);
         }
+        if (arguments.length == 4 && arguments[0].equalsIgnoreCase("pocket")
+                && arguments[1].equalsIgnoreCase("type")) {
+            return purchasePocketBiome(player, arguments[2], arguments[3]);
+        }
         if (arguments.length == 2 && arguments[0].equalsIgnoreCase("visitors")) {
             return updateVisitors(player, state, arguments[1]);
         }
@@ -180,6 +187,59 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
         return true;
     }
 
+    private boolean purchasePocketBiome(
+        Player player,
+        String worldTypeName,
+        String biomeName
+    ) {
+        if (pocketBases == null) {
+            player.sendMessage(ChatColor.RED + "Pocket Bases are currently unavailable.");
+            return true;
+        }
+        PocketBaseState current = pocketBases.state(player.getUniqueId());
+        if (current.level() < 1) {
+            player.sendMessage(ChatColor.RED + "Pocket Base I is still locked.");
+            return true;
+        }
+        PocketBaseBiome biome = PocketBaseWorldType.fromCommand(worldTypeName)
+            .flatMap(worldType -> PocketBaseBiome.fromCommand(worldType, biomeName))
+            .orElse(null);
+        if (biome == null) {
+            player.sendMessage(ChatColor.RED + "That Pocket Base type and subtype are invalid.");
+            return true;
+        }
+        if (biome == current.biome()) {
+            player.sendMessage(ChatColor.RED + "Your Pocket Base already uses "
+                + biome.commandName() + ".");
+            return true;
+        }
+        int price = settings.current().pocketBaseBiomeChangeCost();
+        Material currency = Material.valueOf(
+            settings.current().pocketBaseBiomeCurrencyMaterial()
+        );
+        PlayerInventory inventory = player.getInventory();
+        if (countCurrency(inventory, currency) < price) {
+            player.sendMessage(ChatColor.RED + "Changing Pocket Base biome costs "
+                + price + " " + currency.name().toLowerCase(Locale.ROOT) + ".");
+            return true;
+        }
+        ItemStack[] snapshot = cloneContents(inventory.getStorageContents());
+        removeCurrency(inventory, currency, price);
+        final PocketBaseState updated;
+        try {
+            updated = pocketBases.setBiome(player.getUniqueId(), biome);
+        } catch (IOException | RuntimeException exception) {
+            inventory.setStorageContents(snapshot);
+            player.sendMessage(ChatColor.RED
+                + "The Pocket Base biome change failed; your payment was restored.");
+            return true;
+        }
+        player.sendMessage(ChatColor.GREEN + "Pocket Base biome changed to "
+            + updated.biome().commandName() + " for " + price + " "
+            + currency.name().toLowerCase(Locale.ROOT) + ".");
+        return true;
+    }
+
     private boolean updatePocketMobSpawning(Player player, String mode) {
         if (pocketBases == null) {
             player.sendMessage(ChatColor.RED + "Pocket Bases are currently unavailable.");
@@ -452,7 +512,9 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
         player.sendMessage(colorForPrerequisite(owner.baseLevel() >= 4 && owner.base().isPresent())
             + "Pocket Base " + pocket.level() + ": " + ChatColor.GRAY + price + " "
             + settings.current().pocketBaseCurrencyMaterial().toLowerCase(Locale.ROOT)
-            + " → /basesettings pocket upgrade; mob spawning="
+            + " → /basesettings pocket upgrade; type="
+            + pocket.biome().worldType().commandName() + "/" + pocket.biome().commandName()
+            + "; mob spawning="
             + (pocket.mobSpawningEnabled() ? "enabled" : "disabled"));
     }
 
@@ -485,17 +547,33 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
             List<String> modes = arguments[0].equalsIgnoreCase("visitors")
                 ? VISITOR_MODES
                 : switch (arguments[0].toLowerCase(Locale.ROOT)) {
-                    case "pocket" -> List.of("upgrade", "mobs");
+                    case "pocket" -> List.of("upgrade", "mobs", "type");
                     case "navigation", "flight", "border", "spawnable", "bossbar" -> ENABLE_MODES;
                     default -> List.of();
                 };
             String prefix = arguments[1].toLowerCase(Locale.ROOT);
             return modes.stream().filter(mode -> mode.startsWith(prefix)).toList();
         }
-        if (arguments.length == 3 && arguments[0].equalsIgnoreCase("pocket")
-                && arguments[1].equalsIgnoreCase("mobs")) {
+        if (arguments.length == 3 && arguments[0].equalsIgnoreCase("pocket")) {
             String prefix = arguments[2].toLowerCase(Locale.ROOT);
-            return ENABLE_MODES.stream().filter(mode -> mode.startsWith(prefix)).toList();
+            if (arguments[1].equalsIgnoreCase("mobs")) {
+                return ENABLE_MODES.stream().filter(mode -> mode.startsWith(prefix)).toList();
+            }
+            if (arguments[1].equalsIgnoreCase("type")) {
+                return POCKET_WORLD_TYPES.stream()
+                    .filter(type -> type.startsWith(prefix))
+                    .toList();
+            }
+        }
+        if (arguments.length == 4 && arguments[0].equalsIgnoreCase("pocket")
+                && arguments[1].equalsIgnoreCase("type")) {
+            String prefix = arguments[3].toLowerCase(Locale.ROOT);
+            return PocketBaseWorldType.fromCommand(arguments[2])
+                .map(PocketBaseBiome::commandNames)
+                .orElse(List.of())
+                .stream()
+                .filter(biome -> biome.startsWith(prefix))
+                .toList();
         }
         return List.of();
     }
@@ -557,6 +635,7 @@ final class BaseSettingsCommand implements CommandExecutor, TabCompleter {
     private static void sendUsage(Player player) {
         player.sendMessage(ChatColor.RED + "Usage: /basesettings "
             + "[status|upgrade|pocket upgrade|pocket mobs <enable|disable>"
+            + "|pocket type <void|nether|overworld> <subtype>"
             + "|visitors <allowed|blocked>|navigation <enable|disable>"
             + "|flight <enable|disable>|border <enable|disable>"
             + "|spawnable <enable|disable>|bossbar <enable|disable>]");

src/main/java/games/dmg/spigotbase/PluginSettings.java

@@ -39,12 +39,14 @@ public record PluginSettings(
     int visitorUnlockDiamondCost,
     int pocketBaseUnlockCost,
     int pocketBaseUpgradeCost,
+    int pocketBaseBiomeChangeCost,
     Set<String> baseUnlockMaterials,
     String stoneExpansionMaterial,
     String deepslateExpansionMaterial,
     String obsidianExpansionMaterial,
     String visitorCurrencyMaterial,
     String pocketBaseCurrencyMaterial,
+    String pocketBaseBiomeCurrencyMaterial,
     String pocketBasePortalFrameMaterial,
     Set<String> cooldownExcludedMaterials,
     int bossBarDurationTicks,
@@ -85,6 +87,7 @@ public record PluginSettings(
     private static final int DEFAULT_VISITOR_UNLOCK_DIAMOND_COST = 128;
     private static final int DEFAULT_POCKET_BASE_UNLOCK_COST = 64;
     private static final int DEFAULT_POCKET_BASE_UPGRADE_COST = 16;
+    private static final int DEFAULT_POCKET_BASE_BIOME_CHANGE_COST = 16;
     private static final int DEFAULT_BOSS_BAR_DURATION_TICKS = 60;
     private static final int DEFAULT_NAVIGATION_PARTICLE_COUNT = 5;
     private static final int DEFAULT_TITLE_FADE_IN_TICKS = 10;
@@ -148,6 +151,7 @@ public record PluginSettings(
         requirePositive(visitorUnlockDiamondCost, "visitor-unlock-diamond-cost");
         requirePositive(pocketBaseUnlockCost, "pocket-base-unlock-cost");
         requirePositive(pocketBaseUpgradeCost, "pocket-base-upgrade-cost");
+        requirePositive(pocketBaseBiomeChangeCost, "pocket-base-biome-change-cost");
         baseUnlockMaterials = normalizedSet(baseUnlockMaterials, "base-unlock-materials", false);
         stoneExpansionMaterial = normalizedName(stoneExpansionMaterial, "stone-expansion-material");
         deepslateExpansionMaterial = normalizedName(
@@ -162,6 +166,9 @@ public record PluginSettings(
         pocketBaseCurrencyMaterial = normalizedName(
             pocketBaseCurrencyMaterial, "pocket-base-currency-material"
         );
+        pocketBaseBiomeCurrencyMaterial = normalizedName(
+            pocketBaseBiomeCurrencyMaterial, "pocket-base-biome-currency-material"
+        );
         pocketBasePortalFrameMaterial = normalizedName(
             pocketBasePortalFrameMaterial, "pocket-base-portal-frame-material"
         );
@@ -230,12 +237,18 @@ public record PluginSettings(
             integer(values, "visitor-unlock-diamond-cost", DEFAULT_VISITOR_UNLOCK_DIAMOND_COST),
             integer(values, "pocket-base-unlock-cost", DEFAULT_POCKET_BASE_UNLOCK_COST),
             integer(values, "pocket-base-upgrade-cost", DEFAULT_POCKET_BASE_UPGRADE_COST),
+            integer(
+                values,
+                "pocket-base-biome-change-cost",
+                DEFAULT_POCKET_BASE_BIOME_CHANGE_COST
+            ),
             stringSet(values, "base-unlock-materials", Set.of("GRASS_BLOCK", "DIRT")),
             string(values, "stone-expansion-material", "STONE"),
             string(values, "deepslate-expansion-material", "DEEPSLATE"),
             string(values, "obsidian-expansion-material", "OBSIDIAN"),
             string(values, "visitor-currency-material", "DIAMOND"),
             string(values, "pocket-base-currency-material", "DIAMOND_BLOCK"),
+            string(values, "pocket-base-biome-currency-material", "NETHERITE_BLOCK"),
             string(values, "pocket-base-portal-frame-material", "DIAMOND_BLOCK"),
             stringSet(values, "cooldown-excluded-materials", Set.of()),
             integer(values, "boss-bar-duration-ticks", DEFAULT_BOSS_BAR_DURATION_TICKS),

src/main/java/games/dmg/spigotbase/PluginSettingsValidator.java

@@ -29,6 +29,10 @@ final class PluginSettingsValidator {
         }
         validateCurrency(settings.visitorCurrencyMaterial(), "visitor-currency-material");
         validateCurrency(settings.pocketBaseCurrencyMaterial(), "pocket-base-currency-material");
+        validateCurrency(
+            settings.pocketBaseBiomeCurrencyMaterial(),
+            "pocket-base-biome-currency-material"
+        );
         return settings;
     }
 

src/main/java/games/dmg/spigotbase/PocketBaseBiome.java

@@ -0,0 +1,105 @@
+package games.dmg.spigotbase;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Locale;
+import java.util.Optional;
+
+public enum PocketBaseBiome {
+    THE_VOID(PocketBaseWorldType.VOID),
+
+    NETHER_WASTES(PocketBaseWorldType.NETHER),
+    SOUL_SAND_VALLEY(PocketBaseWorldType.NETHER),
+    CRIMSON_FOREST(PocketBaseWorldType.NETHER),
+    WARPED_FOREST(PocketBaseWorldType.NETHER),
+    BASALT_DELTAS(PocketBaseWorldType.NETHER),
+
+    OCEAN(PocketBaseWorldType.OVERWORLD),
+    PLAINS(PocketBaseWorldType.OVERWORLD),
+    DESERT(PocketBaseWorldType.OVERWORLD),
+    WINDSWEPT_HILLS(PocketBaseWorldType.OVERWORLD),
+    FOREST(PocketBaseWorldType.OVERWORLD),
+    TAIGA(PocketBaseWorldType.OVERWORLD),
+    SWAMP(PocketBaseWorldType.OVERWORLD),
+    MANGROVE_SWAMP(PocketBaseWorldType.OVERWORLD),
+    RIVER(PocketBaseWorldType.OVERWORLD),
+    FROZEN_OCEAN(PocketBaseWorldType.OVERWORLD),
+    FROZEN_RIVER(PocketBaseWorldType.OVERWORLD),
+    SNOWY_PLAINS(PocketBaseWorldType.OVERWORLD),
+    MUSHROOM_FIELDS(PocketBaseWorldType.OVERWORLD),
+    BEACH(PocketBaseWorldType.OVERWORLD),
+    JUNGLE(PocketBaseWorldType.OVERWORLD),
+    SPARSE_JUNGLE(PocketBaseWorldType.OVERWORLD),
+    DEEP_OCEAN(PocketBaseWorldType.OVERWORLD),
+    STONY_SHORE(PocketBaseWorldType.OVERWORLD),
+    SNOWY_BEACH(PocketBaseWorldType.OVERWORLD),
+    BIRCH_FOREST(PocketBaseWorldType.OVERWORLD),
+    DARK_FOREST(PocketBaseWorldType.OVERWORLD),
+    PALE_GARDEN(PocketBaseWorldType.OVERWORLD),
+    SNOWY_TAIGA(PocketBaseWorldType.OVERWORLD),
+    OLD_GROWTH_PINE_TAIGA(PocketBaseWorldType.OVERWORLD),
+    WINDSWEPT_FOREST(PocketBaseWorldType.OVERWORLD),
+    SAVANNA(PocketBaseWorldType.OVERWORLD),
+    SAVANNA_PLATEAU(PocketBaseWorldType.OVERWORLD),
+    BADLANDS(PocketBaseWorldType.OVERWORLD),
+    WOODED_BADLANDS(PocketBaseWorldType.OVERWORLD),
+    WARM_OCEAN(PocketBaseWorldType.OVERWORLD),
+    LUKEWARM_OCEAN(PocketBaseWorldType.OVERWORLD),
+    COLD_OCEAN(PocketBaseWorldType.OVERWORLD),
+    DEEP_LUKEWARM_OCEAN(PocketBaseWorldType.OVERWORLD),
+    DEEP_COLD_OCEAN(PocketBaseWorldType.OVERWORLD),
+    DEEP_FROZEN_OCEAN(PocketBaseWorldType.OVERWORLD),
+    SUNFLOWER_PLAINS(PocketBaseWorldType.OVERWORLD),
+    WINDSWEPT_GRAVELLY_HILLS(PocketBaseWorldType.OVERWORLD),
+    FLOWER_FOREST(PocketBaseWorldType.OVERWORLD),
+    ICE_SPIKES(PocketBaseWorldType.OVERWORLD),
+    OLD_GROWTH_BIRCH_FOREST(PocketBaseWorldType.OVERWORLD),
+    OLD_GROWTH_SPRUCE_TAIGA(PocketBaseWorldType.OVERWORLD),
+    WINDSWEPT_SAVANNA(PocketBaseWorldType.OVERWORLD),
+    ERODED_BADLANDS(PocketBaseWorldType.OVERWORLD),
+    BAMBOO_JUNGLE(PocketBaseWorldType.OVERWORLD),
+    DRIPSTONE_CAVES(PocketBaseWorldType.OVERWORLD),
+    LUSH_CAVES(PocketBaseWorldType.OVERWORLD),
+    DEEP_DARK(PocketBaseWorldType.OVERWORLD),
+    SULFUR_CAVES(PocketBaseWorldType.OVERWORLD),
+    MEADOW(PocketBaseWorldType.OVERWORLD),
+    GROVE(PocketBaseWorldType.OVERWORLD),
+    SNOWY_SLOPES(PocketBaseWorldType.OVERWORLD),
+    FROZEN_PEAKS(PocketBaseWorldType.OVERWORLD),
+    JAGGED_PEAKS(PocketBaseWorldType.OVERWORLD),
+    STONY_PEAKS(PocketBaseWorldType.OVERWORLD),
+    CHERRY_GROVE(PocketBaseWorldType.OVERWORLD);
+
+    private final PocketBaseWorldType worldType;
+
+    PocketBaseBiome(PocketBaseWorldType worldType) {
+        this.worldType = worldType;
+    }
+
+    public PocketBaseWorldType worldType() {
+        return worldType;
+    }
+
+    public String commandName() {
+        return name().toLowerCase(Locale.ROOT);
+    }
+
+    static Optional<PocketBaseBiome> fromCommand(
+        PocketBaseWorldType worldType,
+        String value
+    ) {
+        try {
+            PocketBaseBiome biome = valueOf(value.toUpperCase(Locale.ROOT));
+            return biome.worldType == worldType ? Optional.of(biome) : Optional.empty();
+        } catch (IllegalArgumentException exception) {
+            return Optional.empty();
+        }
+    }
+
+    static List<String> commandNames(PocketBaseWorldType worldType) {
+        return Arrays.stream(values())
+            .filter(biome -> biome.worldType == worldType)
+            .map(PocketBaseBiome::commandName)
+            .toList();
+    }
+}

src/main/java/games/dmg/spigotbase/PocketBaseManager.java

@@ -38,7 +38,7 @@ final class PocketBaseManager {
         PocketBaseState current = states.state(ownerId);
         int nextLevel = Math.addExact(current.level(), 1);
         policy.size(nextLevel);
-        worlds.expand(ownerId, current.level(), nextLevel);
+        worlds.expand(ownerId, current.level(), nextLevel, current.biome());
         return states.updateAndSave(ownerId, state -> state.withLevel(nextLevel));
     }
 
@@ -71,6 +71,27 @@ final class PocketBaseManager {
         return states.updateAndSave(ownerId, state -> state.withReturnPortal(portal));
     }
 
+    PocketBaseState setBiome(UUID ownerId, PocketBaseBiome biome) throws IOException {
+        PocketBaseState current = states.state(ownerId);
+        if (current.level() < 1) {
+            throw new IllegalStateException("Pocket Base I is still locked");
+        }
+        if (current.biome() == biome) {
+            throw new IllegalArgumentException("Pocket Base already uses that biome");
+        }
+        try {
+            worlds.setBiome(ownerId, current.level(), biome);
+            return states.updateAndSave(ownerId, state -> state.withBiome(biome));
+        } catch (IOException | RuntimeException exception) {
+            try {
+                worlds.setBiome(ownerId, current.level(), current.biome());
+            } catch (RuntimeException rollbackFailure) {
+                exception.addSuppressed(rollbackFailure);
+            }
+            throw exception;
+        }
+    }
+
     PocketBaseState setMobSpawning(UUID ownerId, boolean enabled) throws IOException {
         PocketBaseState current = states.state(ownerId);
         if (current.level() < 1) {

src/main/java/games/dmg/spigotbase/PocketBaseState.java

@@ -8,7 +8,8 @@ public record PocketBaseState(
     int level,
     Optional<PocketPortalLocation> entrance,
     Optional<PocketPortalLocation> returnPortal,
-    boolean mobSpawningEnabled
+    boolean mobSpawningEnabled,
+    PocketBaseBiome biome
 ) {
     public PocketBaseState {
         if (ownerId == null) {
@@ -19,6 +20,7 @@ public record PocketBaseState(
         }
         entrance = entrance == null ? Optional.empty() : entrance;
         returnPortal = returnPortal == null ? Optional.empty() : returnPortal;
+        biome = biome == null ? PocketBaseBiome.THE_VOID : biome;
         if (level == 0 && (entrance.isPresent() || returnPortal.isPresent())) {
             throw new IllegalArgumentException("a locked Pocket Base cannot have a portal");
         }
@@ -29,7 +31,9 @@ public record PocketBaseState(
         int level,
         Optional<PocketPortalLocation> entrance
     ) {
-        this(ownerId, level, entrance, Optional.empty(), false);
+        this(
+            ownerId, level, entrance, Optional.empty(), false, PocketBaseBiome.THE_VOID
+        );
     }
 
     public PocketBaseState(
@@ -38,40 +42,63 @@ public record PocketBaseState(
         Optional<PocketPortalLocation> entrance,
         boolean mobSpawningEnabled
     ) {
-        this(ownerId, level, entrance, Optional.empty(), mobSpawningEnabled);
+        this(
+            ownerId, level, entrance, Optional.empty(), mobSpawningEnabled,
+            PocketBaseBiome.THE_VOID
+        );
+    }
+
+    public PocketBaseState(
+        UUID ownerId,
+        int level,
+        Optional<PocketPortalLocation> entrance,
+        Optional<PocketPortalLocation> returnPortal,
+        boolean mobSpawningEnabled
+    ) {
+        this(
+            ownerId, level, entrance, returnPortal, mobSpawningEnabled,
+            PocketBaseBiome.THE_VOID
+        );
     }
 
     public static PocketBaseState locked(UUID ownerId) {
         return new PocketBaseState(
-            ownerId, 0, Optional.empty(), Optional.empty(), false
+            ownerId, 0, Optional.empty(), Optional.empty(), false,
+            PocketBaseBiome.THE_VOID
         );
     }
 
     public PocketBaseState withLevel(int newLevel) {
         return new PocketBaseState(
-            ownerId, newLevel, entrance, returnPortal, mobSpawningEnabled
+            ownerId, newLevel, entrance, returnPortal, mobSpawningEnabled, biome
         );
     }
 
     public PocketBaseState withEntrance(PocketPortalLocation portal) {
         return new PocketBaseState(
-            ownerId, level, Optional.of(portal), returnPortal, mobSpawningEnabled
+            ownerId, level, Optional.of(portal), returnPortal, mobSpawningEnabled, biome
         );
     }
 
     public PocketBaseState withoutEntrance() {
         return new PocketBaseState(
-            ownerId, level, Optional.empty(), returnPortal, mobSpawningEnabled
+            ownerId, level, Optional.empty(), returnPortal, mobSpawningEnabled, biome
         );
     }
 
     public PocketBaseState withReturnPortal(PocketPortalLocation portal) {
         return new PocketBaseState(
-            ownerId, level, entrance, Optional.of(portal), mobSpawningEnabled
+            ownerId, level, entrance, Optional.of(portal), mobSpawningEnabled, biome
         );
     }
 
     public PocketBaseState withMobSpawningEnabled(boolean enabled) {
-        return new PocketBaseState(ownerId, level, entrance, returnPortal, enabled);
+        return new PocketBaseState(ownerId, level, entrance, returnPortal, enabled, biome);
+    }
+
+    public PocketBaseState withBiome(PocketBaseBiome selectedBiome) {
+        return new PocketBaseState(
+            ownerId, level, entrance, returnPortal, mobSpawningEnabled, selectedBiome
+        );
     }
 }

src/main/java/games/dmg/spigotbase/PocketBaseWorldService.java

@@ -4,11 +4,15 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
 import java.util.UUID;
+import java.util.function.Function;
 import org.bukkit.Location;
 import org.bukkit.Material;
+import org.bukkit.NamespacedKey;
+import org.bukkit.Registry;
 import org.bukkit.Server;
 import org.bukkit.World;
 import org.bukkit.WorldCreator;
+import org.bukkit.block.Biome;
 
 final class PocketBaseWorldService {
     private static final int PLATFORM_Y = 64;
@@ -16,23 +20,43 @@ final class PocketBaseWorldService {
     private final Server server;
     private final PluginSettingsProvider settings;
     private final PocketBasePolicy policy;
+    private final Function<PocketBaseBiome, Biome> biomeResolver;
     private final Map<UUID, UUID> ownersByWorld = new HashMap<>();
 
     PocketBaseWorldService(Server server, PluginSettingsProvider settings) {
+        this(server, settings, biome -> resolveBiome(server, biome));
+    }
+
+    PocketBaseWorldService(
+        Server server,
+        PluginSettingsProvider settings,
+        Function<PocketBaseBiome, Biome> biomeResolver
+    ) {
         this.server = server;
         this.settings = settings;
         this.policy = new PocketBasePolicy(settings);
+        this.biomeResolver = biomeResolver;
     }
 
     void loadExisting(Map<UUID, PocketBaseState> states) {
         for (PocketBaseState state : states.values()) {
             if (state.level() > 0) {
+                setBiome(state.ownerId(), state.level(), state.biome());
                 setMobSpawning(state.ownerId(), state.mobSpawningEnabled());
             }
         }
     }
 
     World expand(UUID ownerId, int previousLevel, int newLevel) {
+        return expand(ownerId, previousLevel, newLevel, PocketBaseBiome.THE_VOID);
+    }
+
+    World expand(
+        UUID ownerId,
+        int previousLevel,
+        int newLevel,
+        PocketBaseBiome biome
+    ) {
         World world = ensureWorld(ownerId);
         int newHalfSize = policy.size(newLevel) / 2;
         int previousHalfSize = previousLevel == 0 ? 0 : policy.size(previousLevel) / 2;
@@ -50,6 +74,7 @@ final class PocketBaseWorldService {
                 world.getBlockAt(x, PLATFORM_Y, z).setType(Material.GRASS_BLOCK, false);
             }
         }
+        applyBiome(world, previousLevel, newLevel, biome);
         if (previousLevel == 0) {
             createReturnPortal(world);
             world.setSpawnLocation(0, PLATFORM_Y + 1, 5);
@@ -70,6 +95,12 @@ final class PocketBaseWorldService {
         ensureWorld(ownerId).setSpawnFlags(enabled, enabled);
     }
 
+    void setBiome(UUID ownerId, int level, PocketBaseBiome biome) {
+        World world = ensureWorld(ownerId);
+        applyBiome(world, 0, level, biome);
+        world.save();
+    }
+
     Location arrival(UUID ownerId) {
         return new Location(ensureWorld(ownerId), 0.5, PLATFORM_Y + 1, 5.5, 180.0f, 0.0f);
     }
@@ -114,6 +145,40 @@ final class PocketBaseWorldService {
         return world;
     }
 
+    private void applyBiome(
+        World world,
+        int previousLevel,
+        int newLevel,
+        PocketBaseBiome selectedBiome
+    ) {
+        int newHalfSize = policy.size(newLevel) / 2;
+        int previousHalfSize = previousLevel == 0 ? 0 : policy.size(previousLevel) / 2;
+        Biome biome = biomeResolver.apply(selectedBiome);
+        for (int x = -newHalfSize; x < newHalfSize; x += 4) {
+            for (int z = -newHalfSize; z < newHalfSize; z += 4) {
+                boolean previouslyUnlocked = previousLevel > 0
+                    && x >= -previousHalfSize && x < previousHalfSize
+                    && z >= -previousHalfSize && z < previousHalfSize;
+                if (previouslyUnlocked) {
+                    continue;
+                }
+                for (int y = world.getMinHeight(); y < world.getMaxHeight(); y += 4) {
+                    world.setBiome(x, y, z, biome);
+                }
+            }
+        }
+    }
+
+    private static Biome resolveBiome(Server server, PocketBaseBiome selectedBiome) {
+        Registry<Biome> biomeRegistry = server.getRegistry(Biome.class);
+        if (biomeRegistry == null) {
+            throw new IllegalStateException("Biome registry is unavailable");
+        }
+        return biomeRegistry.getOrThrow(
+            NamespacedKey.minecraft(selectedBiome.commandName())
+        );
+    }
+
     private void createReturnPortal(World world) {
         PocketPortalLocation portal = new PocketPortalLocation(
             world.getUID(), world.getName(), -2, PLATFORM_Y + 1, 0, PocketPortalAxis.X

src/main/java/games/dmg/spigotbase/PocketBaseWorldType.java

@@ -0,0 +1,22 @@
+package games.dmg.spigotbase;
+
+import java.util.Locale;
+import java.util.Optional;
+
+public enum PocketBaseWorldType {
+    VOID,
+    NETHER,
+    OVERWORLD;
+
+    public String commandName() {
+        return name().toLowerCase(Locale.ROOT);
+    }
+
+    static Optional<PocketBaseWorldType> fromCommand(String value) {
+        try {
+            return Optional.of(valueOf(value.toUpperCase(Locale.ROOT)));
+        } catch (IllegalArgumentException exception) {
+            return Optional.empty();
+        }
+    }
+}

src/main/java/games/dmg/spigotbase/YamlPocketBaseRepository.java

@@ -44,7 +44,8 @@ public final class YamlPocketBaseRepository {
                     level,
                     loadPortal(yaml, path + ".entrance"),
                     loadPortal(yaml, path + ".return-portal"),
-                    yaml.getBoolean(path + ".mob-spawning-enabled", false)
+                    yaml.getBoolean(path + ".mob-spawning-enabled", false),
+                    loadBiome(yaml, path)
                 );
                 states.put(ownerId, state);
             } catch (IllegalArgumentException ignored) {
@@ -64,6 +65,8 @@ public final class YamlPocketBaseRepository {
             String path = "owners." + state.ownerId();
             yaml.set(path + ".level", state.level());
             yaml.set(path + ".mob-spawning-enabled", state.mobSpawningEnabled());
+            yaml.set(path + ".world-type", state.biome().worldType().commandName());
+            yaml.set(path + ".biome", state.biome().commandName());
             state.entrance().ifPresent(portal -> savePortal(yaml, path + ".entrance", portal));
             state.returnPortal().ifPresent(portal ->
                 savePortal(yaml, path + ".return-portal", portal)
@@ -87,6 +90,17 @@ public final class YamlPocketBaseRepository {
         }
     }
 
+    private static PocketBaseBiome loadBiome(YamlConfiguration yaml, String path) {
+        String worldTypeName = yaml.getString(path + ".world-type");
+        String biomeName = yaml.getString(path + ".biome");
+        if (worldTypeName == null || biomeName == null) {
+            return PocketBaseBiome.THE_VOID;
+        }
+        return PocketBaseWorldType.fromCommand(worldTypeName)
+            .flatMap(worldType -> PocketBaseBiome.fromCommand(worldType, biomeName))
+            .orElse(PocketBaseBiome.THE_VOID);
+    }
+
     private static Optional<PocketPortalLocation> loadPortal(
         YamlConfiguration yaml,
         String path

src/main/resources/config.yml

@@ -50,6 +50,8 @@ visitor-currency-material: DIAMOND
 pocket-base-unlock-cost: 64
 pocket-base-upgrade-cost: 16
 pocket-base-currency-material: DIAMOND_BLOCK
+pocket-base-biome-change-cost: 16
+pocket-base-biome-currency-material: NETHERITE_BLOCK
 pocket-base-portal-frame-material: DIAMOND_BLOCK
 
 # Spawnable block overlay