Sinkholes are basically large holes or depressions in the ground that appear when the land surface collapses into cavities below. They are usually formed through erosion caused by groundwater. Sometimes it can take thousands of years of erosion before they finally collapse. As such, some sinkholes are really special places where lots of amazing discoveries have been made—both on dry land and underwater.
Click through to find out what surprising discoveries were made in sinkholes.