Creating a Tunnel in Terrain


I started working on a map and cannot find how to create a cave with 2 exits in a mountain (exit on each side and enlarged cavity in the middle)

the problem is that creating simple objects and connecting each side to the terrain is difficult due to a lot of edges,
and there is no tool for extending mesh (apart from Grab, which makes non-dividable mesh for continuing)

Is there a way to make a simple tunnel in terrain ?

submit your blend file.

there are 100+ ways to make a tunnel, the most effective way depends on your mesh.

most simple would probably be like this.