A rare type of cave formation, balloons are a small, gas-filled pouch usually made of hydromagnesite. Their origin is not completely known, but is likely related to moonmilk, a material of high plasticity. Balloons probably occur when solutions under pressure seep into a cave through cracks or out of porous walls of limestone. If they meet moonmilk on their way out, the material may expand much like a rubber balloon. The thin moonmilk coating which forms the cast may crack or dry out, exposing the underlying hydromagnesite balloon.

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Created:December, 1998
Author: Dave Bunnell