I have to agree....impressive. @sp1187 does touch upon his plumbing magic in his build thread HERE, but I'd also like to hear more detail. Very impressive modelling skills.
This stuff right here sticks to PVC. It's a modeling epoxy used to make fake rocks and back drops in zoo and aquarium displays. I'm sure it's used for a million other things. But I know for a fact coral grows in this stuff. It's not the cheapest way to make a scape, but if you get creative you can do some incredible pieces. Smudge it on and press it with your hands into the shape you want. Then use pieces of dry rock to press a rock texture into it. You can also finish it with chipped dry rock, ie press it in like you would sand. Or even stick a nice piece of real rock into it .