I can vouch for the truth of that statement! Thirty plus years ago I was on a canoe trip down the Wacissa River near Tallahassee, FL. We were not familiar with the river and took a wrong turn that led us off along a tributary of some sort. Everything looked fine to us until we rounded a little curve and realized it looked like the waterway ended just ahead. Hmmmm . . . There was all sorts of trash and debris floating on the surface and as you got closer you realized it was moving. Around. In ever tighter circles! Wow, talk about pucker factor! We realized we were looking at a huge whirlpool in the river!
Turns out the tributary we had wandered onto went underground there to a cave and the water going in ends up in the Gulf of Mexico some miles west of there.
That was scary.
In 1980 somebody pulled the plug on Lake Peigneur. (an oil rig drilled into a salt mine under the lake)
http://en.wikipedia.org/wiki/Lake_Peigneur