Zorbing is the recreation or sport of rolling downhill in 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. There are two types of orbs, harnessed and non-harnessed. Non-harness orbs carry up to three riders, while the harness orbs are constructed for one to two riders. Double-harness spheres have different slope requirements, and must only be operated in specific locations. The longer runs are approximately half a mile. The first zorbing site was established in Rotorua, New Zealand, by David and Andrew Akers.