I want to build a game like the original Descent game where the map was tunnel-based. Is that possible using unity? I’ve never done this before and need some help getting started. Thanks! 
Why wouldn’t it be possible? Just model a tunnel map and you’re good to go.
sorry… I’m a total newb… do you mean I would make a tunnel map as an object in another program and import it into the level editor? Or do I build it in Unity3d? (I saw another post talking about making a half-pipe-like trench and then putting a mesh over top for the other half to produce the tunnel. That sounded like too much work…)
I would suggest modeling the level in a program such as Blender and importing it into Unity. To take it a step further, you could model modular pieces of a level that fit together and import those into Unity to assemble any level right in the editor.
I think that something like GameDraw or ProBuilder would be very useful for making this kind of map directly in Unity.
Blender is free though…