Resin. Use some cheap resin(polyester, fiberglass resin), mixed with cooking flour. That will make it sticky and thick, so just coat the underside of the terrain and it will be light and strong.
Either that or pva mixed with sand. Actually, this would be a far easier way of doing it, plus pva/sand mix has a good amount of flex in it. Dont fill the back of the piece, just give it a good thick pasting of the stuff.