India, Sept. 28 -- Mammoth Cave is widely recognized as the longest known cave system on Earth, with more than 426 miles of mapped passages. But the more surprising story is what happens outside the cave. Rain miles outside the park boundary can soak into the ground and run through hidden passages before showing up inside the cave itself. For decades, hydrogeologists Chris Groves, Joe Meiman and Shannon Herstein have tracked exactly how this hidden water network behaves in research published in the U.S. Geological Survey Karst Interest Group Proceedings.

You can't see the groundwater moving. It moves through cracks and veins far under the ground, out of sight. So scientists have devised a clever workaround: dye tracing. They pour a harml...