Actually, I've been scouring the countryside for the last week looking
for really thin shards of slate or shale to goo to the back of the
tank. I have an idea in my mind of creating a labyrinth look with a
thin rock wall. Onyx and black sand on the bottom, a frame that
covers the bottom of the tank so you can't see the substrate, you get
what I mean? Problem is, so far can't find the right rock. What
little I did find is too thick and heavy to go on the back glass.
Maybe if I could find a way to split it thin enough...

I know you're idea, and NetMax probably has your solution, cheap,
lightweight and pretty fun if you like the DIY route....
http://www.csd.net/~cgadd/aqua/art_o...tmax_styro.htm

another guy in the newsgroup, just shared pics of his tank, Mike Noren, very
nice looking background that you may be interested in too....
http://hem.bredband.net/micnor/mitt_akvarium.htm

Advantage to this route, weight... your tank will not suffer from the rock
weight, styro wont' affect your pH, and man does it look great... you can
do whatever YOU want to do....

But the idea of caves, I know what you mean, around here slate is 5.99 a
lb.... pretty pricey for me.. so I opted another route... but will be
revisiting this soon....