Terrain Collision
Terrain uses heightfield collider per terain patch. You can specify the Collision LOD that can be used to reduce collision quality but improve simulation performance due to lower resolution of the heightfield. Each terrain can have a custom physical material and use heightmap holes mask to create caves and tunnels under the terrain.
If your game uses runtime-generated terrain then ensure to enable Support Cooking At Runtime option in Physics Settings to support generating heightfield data at runtime.