They're called lawn blisters or grass waterbeds, and they occur when water builds up beneath the surface of the grass. It creates a phenomenon where the grass acts much like a waterbed as the water trapped beneath causes the surface to jiggle around.
For one Alabama family, the temptation to pop the lawn blister proved too much. Video shot by Aimee Joy Childers shows her two sons hopping around on the grass waterbed until it popped, sending muddy water gushing up to the surface as her boys giggled.
Childers said the bubble formed after several days of heavy rain in the area.