It's called Great Stuff, it's expanding foam. I only used 2.5 cans of it, but I've seen some people on here use much more than that to create really awesome faux rock walls! It's lightweight, but solid and it sticks well to the plywood.Nice!! I was following your thread because I too am interested in building a custom habitat. When you mentioned building the wall my mind started running boy! Lol and this is exactly what I imagined!! But I have never built a wall with cliffs like this, what is that opaque substance you used to create the ledges?? Holding the pots in place.