How can I hide terrain above a certain elevation?

Hello,

I'm trying to make a base building mechanic where the base is constructed on a grid with different levels of elevation. A good example of this is the screenshot below.

Underground this was pretty easy, but now that I am looking at the above ground portion I'm a bit stumped.

I want to use some kind of plane object like unity terrain to represent the above ground portion but mask the portion of the mesh that is above a certain y value. I've been experimenting using a depth mask shader and a cube, but when I successfully mask I can't see the terrain underneath the cube. I only want to mask what is inside. Does anyone know of a trick to accomplish this? I'm out of ideas and search queries.