Underground tunnels and caves

Any advice or tips on how to do this easily?

Terrain can not cover a cave or tunnel. Terrain can only move up or down, and never over itself, and you can not successfully build below the terrain surface and be able to access the area normally. So all caves and tunnels start with a trench or a pit.

Terraform a trench or pit. Line the sides and top with prims. If you look in your Library, there are some large jagged rocks that are made by cutting and twisting and tapering a cube prim. Those, when streached varuious ways, make great walls and floors for natural caves.

Using sculpted prims can make more realistic cave walls and ceilings. But remember that they have to be phantom, so you still need other prims to at least roughly aproximate floors and walls.

thanks for the advice