A rare type of cave formation, balloons are small, gas-filled pouches 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.
AUTHOR: Dave Bunnell