Mammoth Cave's miles of hollow halls were already thousands of years old when the first human beings came on the scene. Native Americans of the Early Woodland period gathered minerals from Mammoth Cave between 2,000 and 4,000 years ago, and objects they left behind – slippers, cane torches, gourds, and mussel shells – remain perfectly preserved in the cave.
There are 25 entrances to the cave, 15 of which are natural entrances, mainly utilizing sinkholes.
An ancient sea that covered the central United States 325 million years ago laid down over 600 feet of soluble limestone here which was then covered by a sandstone and shale cap deposited by an ancient river. The sea and the river disappeared, and erosional forces eroded the cap until about 10 million years ago when cracks and holes began to expose the limestone underneath. Rainwater worked its way underground in the form of sinking streams which began hollowing out the cave as underground rivers. This photo from the park service shows a ranger leading a tour group just after entering the cave.
The national park was established in 1941 to protect the unparalleled underground labyrinth of caves, as well as the rolling hill country above and the magnificent Green River valley. Since then, ongoing study and exploration have shown the park to be far more complex than ever imagined, hosting a broad diversity of species living in specialized and interconnected ecosystems. The park's challenge is to balance these remarkable and sometimes fragile living networks with the public's enjoyment of them.
The park contains several species of endangered plants and animals, including Eggert's Sunflower, the Eyeless Cave Shrimp, and several species of river mussels, among others. Creatures that spend their entire lives in Mammoth Cave adapt to the dark world. Some types of cave fish, for example, do not grow eyes – supporting these extra unnecessary organs would consume precious energy in their nutrient-poor environment. Mammoth Cave National Park is home to over 70 threatened, endangered, or state listed species. These species include birds, crustaceans, fish, gastropods, insects, mammals, mussels, plants and reptiles. The park was declared an International Biosphere Reserve in 1990.
In 1841, cave owner Dr. John Croghan believed the cave air might cure his patients suffering from tuberculosis. He brought 16 patients into Mammoth Cave that winter and housed them in stone and wood huts. After some perished, they left the cave, for of course the cave air offered no cure.
Stalactites grow downward and seem to hang "tight" to the ceiling, while stalagmites grow upward, and in the future, they "might" reach the ceiling someday.
Mammoth Cave's formations include many types of calcite formations, but even without the world's longest cave system, the land encompassing the ground above Mammoth Cave would merit its national park status due simply to its extraordinary density and diversity of plant life. While the country's acknowledged "showcase of vegetation" is Great Smoky Mountains National Park which has approximately 1,500 flowering species in its more than 500,000 acres, Mammoth Cave National Park supports more than 1,300 species in only one-tenth of that acreage.
Visitors to the park hike and ride horses on more than 70 miles of surface trails, fish and canoe in the Green River, camp, and picnic. In fact, I have backpacked for four days through its magnificent wilderness, and even though it was spring vacation and the flora had not blossomed out yet, it was still a beautiful locale to explore. Below are two photos from that backpack trip, and the trip report is here.
The Green River and the valley it flows through are quite lovely even in the winter and early spring months.
Recently, a bike trail was built utilizing parts of the old rail line that brought visitors to the caves. I biked it in 2017 and posted photos and info about the trail here.
I also remember visiting the cave in 1962 with my mom and sister as well as my aunt, uncle, and two cousins. We took a tour that included a boat trip on an underground river. Decades later when I returned, I discovered the tour no longer was offered. The Echo River Tour was discontinued in the early 1990s because the introduction of daily human traffic down in the river levels of the caves was causing harm to aquatic creatures that live there. Also, keeping the lower passages (which flooded frequently) open for the public was prohibitively expensive. I understand that in season, they still do offer the River Styx Tour which allows visitors to get a glimpse of the underground rivers. I also found this photo below, which was taken of entire tour group back in 1962. My family and I are in the front two rows.