We're currently creating a cave/tunnel "taj mahal". It starts with an egg crate bottom, pretty slabs of rock glued to the egg crate, then a black pvc elbow covered in tumbled petrified rock all over and around it. Then there's an elevated section back behind the pvc for two clay pots with plants all surrounded by the tumbled stones and filled in with flourite for the plants.
This all sits on a pretty massive sized slate stack that has two large caves divided by a smaller petrified wood "stump" and surrounded by more stacked slate structures.
My husband is a home builder and frustrated architect - so it all has to look like something he could "claim" as his creation. Hate to have to admit it since I didn't create it, but it's going to be awesome. Very natural looking I believe. But since we're doing a "peaceful cichlid tank", pots for the plants scattered about is a must.