Zorbing is the recreation or sport of rolling downhill inside an orb, generally made of transparent plastic. Zorbing is generally performed on a gentle slope, but can also be done on a level surface, permitting more rider control. In the absence of hills some operators have constructed inflatable, wooden, or metal ramps. Due to the buoyant nature of the orbs, Zorbing can also be carried out on water, provided the orb is inflated properly and sealed once the rider is inside. "Water walking" using such orbs has become popular in theme parks across the UK.