What is a sinkhole? Far from being just a hole in the ground, a sinkhole is a very special type of natural phenomenon. Born of the interaction between water and rock, sinkholes appear anywhere rock dissolves easily. Some sinkholes happen gradually over hundreds of years. Other sinkholes open up almost instantly. These are the ones you hear about in the news--the sinkholes that swallow houses and chew holes in highways. But a sinkhole is much more. It can fill up with water to become a pond or lake. It might serve as a drainpipe for surface water to flow straight into the ground. It may swallow a river or become a spring. It can open up a new entrance to a cave. In all cases, sinkholes provide a cool, damp environment, supporting plants and animals different from those in the habitats surrounding it. Curious explorers will appreciate the suggested parks, preserves, and forests in which they can visit a sinkhole.