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The Alien Trampoline, also known as the Sphincter, is a Xen floral-like mound propelling anyone or anything, including displacement fields, that touches it into the air whilst hissing as if air is escaping.
It has two variants introduced in Half-Life. One first seen in Black Mesa's Topside Motorpool, which appears to have developed on grounds. The other variant, which is known as a Blowhole, is first seen on Xen contained in a small hole shut by an aperture-like cover. It opens and closes, allowing any being to fall into the hole. When the aperture opens, the being is propelled outside of the hole. If stuck, it will affect its health.
 Behind the scenes
- “One of the cleverer constructions in Xen, our alien dimension, is a series of puckered orifices which swallow you and spit you high into the sky for a lethal fall. These were picked up by one of the other designers and worked back into the terrestrial levels, giving the impression that the earthly plane is being infested by the alien one. Then a third designer did more benign earthly versions which fling you one or two stories high. So the puckers turned from a localized trap into an important gameplay element for a significant part of the game. This kind of borrowing and scattering of imagery is a function of revision, and to me one of the most effective things that happened spontaneously in the latter stages of Half-Life's creation.”
- — Marc Laidlaw