The world's largest system of caves

Beneath the forest covered hills of southern Kentucky, USA are the MAMMOTH CAVES, the world's largest system of caves. There are about 570 km of underground passageways on 5 different levels. These have huge chambers, vertical shafts and splendid forms of gypsum flowers and other rare natural wonders. The temperature here is always cooler than outside in summers and warmer in winters.
Some of these passages end into rooms big enough to hold a nine storey building!