Mammoth Cave National Park is located in Mammoth Cave, Kentucky. It is the world's largest cave system totalling nearly 350 miles of underground passages. This makes it almost three times longer than the next longest cave system, located in Russia. It was established as a national park in 1941 but exploration of the caves had been going on for hundreds of years and tours since the 1800s.
Mammoth Cave was formed by the erosion of limestone by water. At first, the area of Mammoth Caves was covered by a giant sea. Over time, the sea disappeared and freshwater seeped into cracks in the ground. The water eroded out these cracks creating large underground rivers. After hundreds of thousands of years, the rivers dug their way down deeper into the earth leaving behind the large caves they had created. So, the existing caves are basically dry while the river continues to dig deeper down in the earth. The result is much different from the traditional stalactites and stalagmites found in wet caves.
Tours of various difficulty are offered. These tours range from easy one hour hikes through the caves to an extremely strenuous five hour crawl through dark, muddy passages. I took the four hour walking tour which covered about three miles of the largest caves. There really aren't words to describe how big everything is. Some passages are a hundred feet wide and half a mile long while others are tiny holes through the rocks. Some of the limestone chunks that have fallen from the ceiling weigh tens of tons. One of the strangest parts is the restaurant in the middle of the tour where you stop for lunch. Still, tour sizes are limited so it's not such a tourist spot as some other caves tend to be.
Up on the surface, Mammoth Cave has a hotel to stay in as well as some very nice campgrounds (despite the fact that it's set up for car camping). There's almost one hundred miles of hiking trails as well. Several endangered species live in and around the caves. Deer are all over the place - watch out when you're driving at night!
Mammoth Cave is truly a remarkable place to go. If you go during one of the off-peak times of the year, it's very quiet and not crowded at all in the campgrounds. In addition, the tours will be smaller and with fewer children. The cave itself is absolutely amazing. I can't even begin to describe it with only words. You have to see if for yourself. Finally, don't go into town unless you have to. It's not such a bad place but is really touristy.
This information taken from the appropriate section of the NPS website (http://www.nps.gov) and my own memories of the tour. Much more information is available if you go there yourself.