Mammoth Cave has a way of drawing people in.
A cold wind rises from the cave entrance and envelops the uninitiated.
“The first time I entered the cave, what struck me was that immediate visceral blast of cold air. You know a change is about to occur,” Davis McCombs says.
McCombs first descended into the earth at Mammoth Cave at the age of seven, on a tour with a grade school class. He came home obsessed. After 9 years as a seasonal ranger at Mammoth Cave National Park in central Kentucky, giving tours is no longer enough for McCombs. Now he writes poetry about this designated World Heritage Site which makes up the world’s longest underground network of natural tunnels
McCombs’ first book, Ultima Thule, is named after the rocks that were thought to mark the end of the cave system until 1908, when explorers found a small crawlway beneath and the rest of the cave beyond. His work won the 1999 Yale Younger Poets Contest and was recently published by Yale University Press.
It is the intricate labyrinth of tunnels, the mystical geologic and historic forces that shape them, and their fragile, dark loving inhabitants that inspire his writing.
A River Below the Surface The Green River carved this underground world out of limestone over millions of years.
The rock itself began forming 350 million years ago, something McCombs explains daily when he gives tours. “But as much we talk about it, we can’t really understand that. It is so much greater than our own life spans,” McCombs says.
To make a cave you need rock that will dissolve, and a way for water to flow through the rock, Joe Meiman, a park hydrologist, explains. Once water enters the rock, it wends its way through crevices in the soluble limestone, slowly enlarging tiny fractures into an underground riverbed. After at least 2 million years of water flow, the large caverns of the uppermost layer of the cave were formed, according to a new dating technique based on radioactive decay in quartz developed by Darryl Granger, a geology professor at Purdue University in Indiana.
Then the water table shifted downward, and began forming another level of the cave beneath the first. Mammoth Cave has four major levels, each descending deeper into the earth. Water runs through the rock beneath, where the cave is still forming.
McCombs writes of this etching process in The River and Under the River, “If I put my ear to the ground/ could I hear the drag of the river turning/ limestone into silt?”
A cap of more slowly eroding sandstone protects the upper levels of the cave from wearing away themselves, helping make this the largest recorded cave system in the world.
“Mammoth cave is immensity on the loose,” McCombs says. “My grandchildren will still be finding cave out here.” Over 350 miles of cave have been mapped so far, and some speculate the cave could have over 600 miles of corridors.
“No one has ever come/ this far — a dusty, Hell-bent crawl, past pits/ and keystones, to find myself deep in the ridge./ I was drawn to wonder, the margins of the map./ Breath and a heartbeat. A fading lamp./ I was coffled to the light,” writes McCombs in his poem Ultima Thule.
While hundreds of people wander this vast cave on tours, Meiman often works on his research in silence, just miles away in the same cave network.
A World ApartBut the cave is not totally silent. Meiman says subtle sounds, colors and weather patterns fill the cave, especially at the lowest levels, over 300 feet below the surface, where water wears at the limestone. “It makes you more aware of light, sound, shadows,” Meiman says.
Winds like the one that greeted McCombs on his first visit often billow through the tunnels. Explorers can use these cave breezes to learn about what lies ahead, something McCombs wrote about in his poem, Cave Wind: “knowing/ its speed portends the cave we'll discover, whether we will/ walk or crawl, the breadth/ of its breath, its given.”
As Art Palmer, a hydrology professor at State University of New York-Oneonta who has studied Mammoth Cave for over 30 years, points out — if you are in a cave tunnel with air blasting through it, you know it goes somewhere. Breezes can form as cool air settles to the bottom of connected cave tunnels. This causes air to shift throughout the cave network. Local storms also lead to pressure changes at the surface that can create breezes within the cave.
Like many caves, Mammoth cave stays at a pretty constant 53 degrees Fahrenheit, and is extremely humid. When a heavy summer rain brings a rush of warm water into the cave, the moist cave air condenses and an eerie fog forms in cave tunnels near the underground river and cave openings. During such fogs, Meiman imagines the early explorers in the cave, not understanding why fog had suddenly clouded their lantern light.
Floods bring more than just fog. The cave environment is very low in nutrients, explains Tom Poulson, a biology professor from the University of Illinois. Occasional large floods renew the organic matter that feeds crustaceans and plankton, which in turn feed larger cave species like blind cave fish.
“Cave creatures live slow, live long, and reproduce seldomly,” Bill Pearson, a biology professor at the University of Louisville, Kentucky says. They also have enhanced senses to maneuver in the dark. Cave crayfish live well over 100 years, while their counterparts on the surface live only 10. Pearson says the often illusive, nearly colorless cave fish standout against the darkness of the cave when a beam of light catches them.
“Touched,/ they said, fish with no eyes! until I sloshed/ a pailful into light, reveled in their silence,” McCombs writes of these creatures in Bottomless Pit.
Cave Connection The cave’s presence alters the surrounding landscape as well. Water doesn’t typically flow in streams on the surface, but seeps underground through sinkholes, or bowl shaped depressions, Meiman says. The landscape beyond the sandstone cap is riddled with sinkholes that feed water down into the cave.
Sinkholes often plug up, and form ponds that can cover an acre or two of land, Palmer says. Heavy rains can shift the soil that plugs the bottom of these ponds, opening the sinkholes again, and causing these ponds to disappear overnight, as McCombs describes in his poem Ponds. “In Caveland, every pond's a fluke./ Let them be brief, then, as the land/ gives up the ghost of fog, morning.”
“We take it for granted here that the land is unstable,” McCombs says, “it never lets you forget it is combed by a vast network of cave.”