From Minetest Developer Wiki
The map generator will generate a world according to the mapgen flags that are set for it.
This is a list of currently supported flags:
- trees: generate trees
- caves: generate caves
- v6_biome_blend: blur out biome edges
- v6_jungles: generate jungles
- dungeons: generate dungeons
- flat: make a completely flat map
They are set in minetest.conf before creating a new world.
An example configuration (the default):
mg_flags = trees, caves, v6_biome_blend