Unity 3D indoor scene help?! Thread

So I started using Unity 3D, and I can use it pretty well for a beginner - but one thing I can't seem to do is to create indoor scenes. After creating the outdoor environments, I can't seem to find anything which would allow me to create indoor-type areas, such as caves, sewers, castle corridors, ect. Can someone help me? Just a simple guide on how to create a fairly detailed indoor environment would be extremly helpful

You can definitely download a lot of place holder models for ideas of where to place things before to make everything you need.  If you want Caves then you're kinda of in a pickle if you want them to come out of the terrain.  There is actually a sewer test project you can download from the unity store.  You can see how that one works out.