Every watershed is different. In some places water can make a little river under the ground that we can't see. When that happens, there is usually a cave nearby!
Sometimes water under the ground can go through big rocks that clean out all the bad stuff. Whenever water gets cleaned out underground like that, we call it an aquifer. That's where most of our drinking water comes from.
Not all underground water gets cleaned though, even in caves! Caves are like big, open tunnels for water, and sometimes water in a cave can go through really fast and never get cleaned out. When water does not get cleaned, we call that a Karst system.
Karst systems have underground streams that we can't see, and sometimes the ground over the hidden stream falls apart and leaves a hole behind. Those are called sinkholes, and some people find caves by looking for lots of sinkholes.
