public class Test extends JavaPlugin {
    private ChunkGenerator chunkGen;

    @Override
    public void onEnable(){
        chunkGen = new ChunkGeneratorWorld();
        World nether = WorldCreator.name("test_nether").type(WorldType.FLAT).environment(Environment.NETHER).generator(chunkGen).createWorld();
        Bukkit.getLogger().info("DEBUG: The world height is " + nether.getMaxHeight());
        Bukkit.getLogger().info("DEBUG: The block type is " + nether.getBlockAt(0, 250, 0));
    }
}

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public class ChunkGeneratorWorld extends ChunkGenerator {

    @Override
    public ChunkData generateChunkData(World world, Random random, int chunkX, int chunkZ, ChunkGenerator.BiomeGrid biomeGrid) {
        ChunkData c = createChunkData(world);
        c.setBlock(0, 250, 0, Material.GLOWSTONE);
        return c;
    }

    @Override
    public boolean canSpawn(World world, int x, int z) {
        return true;
    }

    @Override
    public List<BlockPopulator> getDefaultPopulators(final World world) {
        return Collections.emptyList();
    }
}