Mammoth Cave in Kentucky is the longest cave system known on Earth. Explorers have already mapped over 686 kilometres of passages, and surveyors keep adding more every year. The next-longest cave isn’t even half as big.
The cave formed because the area sits on a thick layer of limestone with a layer of harder rock on top. Slightly acidic rainwater seeped through cracks for millions of years, dissolving the limestone and hollowing out a maze of underground tunnels stacked on five different levels.
People have explored Mammoth for thousands of years. Native American footprints, torches, and even mummies have been found in deep chambers. The cave is now a national park, but only a fraction of the passages are open to tourists.
