In the Disney film The Rescuers, what actually is the black hole?

In the 1977 Disney film The Rescuers, Penny is made to go down a hole in the ground into a cave to find a diamond. But the cave has a huge, deep hole that spurts out a load of water occasionally (lovely source of nightmares for 7-year-old me). I was just wondering what this actually is? Some kind of sewage, or what?

Here’s the scene.