Bounce Below is in Llechwedd Slate Cavern in North Wales, and they have filled it with three levels of bouncy nets which act like trampolines/bouncy castles! We booked our tickets a few days before hand and when we arrived we just had to collect a helmet and ride in the little train into the cavern.
There are tunnel ladders and shoots to get from level to the other. The shoots were slightly terrifying as they seemed to be an almost vertical drop when you first go down and the netting wraps tightly around you. But it was lots of fun. They looked like they were constructing a new section to be open in Spring 2015 and there is also a zip wire so we may have to take a trip back again next year!