At the start of the tour
Welcome to Historic Entrance and River Styx Spring
Welcome to Mammoth Cave National Park. Somewhere under this hillside runs the longest known cave system in the world. Explorers have surveyed and mapped more than four hundred and twenty-six miles of passages, and they're still finding new ones. The cave lies in limestone that formed under a warm, shallow sea more than three hundred million years ago.
Today's walk is short. We'll go down the hill to the Historic Entrance, the cave's famous natural doorway, and then along the River Styx Spring Trail to the place where an underground river comes out of the cave. It's about half a mile one way, steep in places, and the walk back is all uphill. I'll tell a story at each stop and stay quiet in between.
A few things before we start. This walk needs no ticket, but the cave itself is entered only on a ticketed tour, to protect the cave and to keep you safe. Stay on the trail. Ticks, poison ivy and two kinds of venomous snakes, the copperhead and the timber rattlesnake, share these woods, so watch where you put your hands and feet. And please leave every rock, plant and artifact exactly where it is.
Point 1 · At the stop
Welcome to Mammoth Cave
We're starting just below the visitor center. It's where cave tickets are sold and where every cave tour begins, and inside there's a museum about the park's history and geology, and a short film about exploring the cave.
Keep one thing in mind as we walk: the cave isn't only down at the entrance. It runs under this whole ridge, buildings and all. Right beneath the lodge next door is the Rotunda, one of the largest rooms in Mammoth Cave and one of the most visited, where the remains of the old saltpeter mining operation still stand in the middle of the floor.
Around the visitor center, the park has put up Beneath Your Feet signs at fourteen spots along the trails and walking paths. To place them, scientists used a cave radio. A surveyor deep in the cave sends out a magnetic signal, and a team up on the surface follows it with a receiver until they're standing directly over that spot. So if you pass one of those signs, it really is telling you what lies under your shoes.
When you're ready, let's walk down the hill to the place where the cave meets the daylight.
History, on the way there
On the way: The Historic Entrance
Now for the Historic Entrance. People have been walking into the dark here for a very long time. Some four to five thousand years ago, Native Americans of the Late Archaic and Early Woodland periods were exploring Mammoth Cave. They lit their way with torches of river cane, and with mussel shells carried up from the Green River they scraped soft mineral crusts like gypsum off the passage walls. In all they found more than nineteen miles of passages. Their torches, woven sandals and gourd bowls still lie in the dry passages. Around two thousand years ago, the exploring and mining faded away. Today the park consults with seven Cherokee, Shawnee and Chickasaw nations that are traditionally affiliated with this land.
Settlers came to the Green River valley in the 1790s, and they found their own use for the cave. Its dirt held saltpeter, the main ingredient of black gunpowder. During the War of 1812, enslaved men mined vast amounts of it here, working long hours in smoke, cold and darkness, helping to defend a freedom they themselves did not have, and the saltpeter was shipped east to the gunpowder makers.
Point 2 · At the stop
The Historic Entrance
This is the Historic Entrance, the cave's most famous doorway. Cave tours still come and go this way, as they have for nearly two centuries, and inside it stays around fifty-four degrees all year. In 1996 the park fitted this entrance with bat gates and airlock doors, and today everyone leaving the cave wipes their shoes on a special mat, to slow white-nose syndrome, a fungal disease that is killing bats.
The first known formal tours began in 1816. In 1838, an enslaved teenager named Stephen Bishop began leading visitors in from here, with other enslaved guides including Mat and Nick Bransford. Stephen crossed a chasm called the Bottomless Pit, opening miles of new cave, drew a map of the passages from memory, and called this a grand, gloomy and peculiar place. He was freed in 1856 and died the next year, and his grave is in the Old Guide's Cemetery, just off the Heritage Trail. Mat's family guided here for generations. When the Park Service took over in 1941, none of the Black guides were asked to stay. It was 2004 before one of Mat's great-great-grandsons came back as a park guide, to lead tours through the cave his ancestors knew.
History, on the way there
On the way: Water, rock and five levels of cave
Now for the water, because water made this cave. Rain soaks through the soil and picks up carbon dioxide, which turns it into a weak acid. That acid works its way into cracks in the limestone and slowly dissolves the rock, widening the cracks into channels, and the channels into passages. Mammoth Cave's passages began forming about ten to fifteen million years ago, when streams on the surface started sinking into the rock.
The ridges here have a lid. Their top layer is sandstone, which resists erosion, and under that is a layer of shale, which water can't easily pass through. That roof of shale is one of the reasons so much of Mammoth Cave stays dry inside.
Point 3 · At the stop
Water, rock and five levels of cave
Inside these ridges, the cave is stacked in floors. As the Green River cut its valley deeper, the streams inside the cave followed it down, abandoning their old, higher channels and carving new ones below. Today there are five levels: four dry ones, and the river level at the bottom, more than three hundred feet beneath the surface, where water is still carving cave.
Mapping all of it takes patience. Volunteer survey teams crawl the passages with a compass, a tape or a laser, and an instrument for slope, measuring from one point to the next like connect-the-dots. In 1972 one team found the connection between this ridge's cave and the Flint Ridge system next door, making it the longest known cave in the world, at a hundred and forty-four miles. It has roughly tripled since, and geologists think there could be six hundred more miles still unmapped.
The lowest level still floods. On the Historic Tour route, a passage called River Hall is the only toured part of the cave that fills when the river inside rises, and in 1937 the water there was recorded at fifty-seven feet.
History, on the way there
On the way: River Styx Spring
Now for the spring, and the life of the cave's rivers. Mammoth Cave's lowest passages hold real rivers, with names like the River Styx and the Echo River. For years visitors rode boats on the Echo River, but those tours ended in the early 1990s, partly because the traffic was harming the creatures that live in the water. In season, one cave tour still offers a glimpse of an underground river.
Those creatures are strange and rare. The northern cavefish was first discovered in Mammoth Cave's rivers. It has no eyes and no color, and it finds its way with rows of sensitive bumps on its skin. Early guides used to catch these fish and sell them to visitors as souvenirs. The Kentucky cave shrimp, an endangered species, lives only in the lowest levels of the caves right around here, and every one of these animals depends on clean water seeping down from the surface.
Point 4 · At the stop
River Styx Spring
This is River Styx Spring, where the underground river of the same name leaves the cave and flows a short way out to join the Green River. Rain that falls on the sinkhole plain to the southeast sinks underground, finds its way through the cave's rivers, and comes out at springs like this one, all along the Green.
The Green River is the reason the cave is shaped the way it is, and it's a treasure of its own. It runs through the park for about twenty-seven miles and is one of the most biologically diverse rivers in North America, with more than eighty kinds of fish and some of the nation's rarest freshwater mussels. For more than a century, a lock and dam on the Green held the river higher, and even raised the water inside parts of the cave, until a breach in 2016 and the dam's removal the next year.
That's the end of our walk. The first explorers with their cane torches, the miners, and the guides who learned this cave by heart all came to the same dark doorway on this hillside. Take your time on the climb back up, and listen for the water as you go.