At the age of five, Roger Brucker made an important discovery: his new trainers had enough grip to scale his friend’s laundry chute. He promptly climbed from the basement to the first storey bathroom. Regrettably, it was occupied.

Mammoth Cave – est 1941 Kentucky Roger Brucker, cave explorer Read more about BBC Travel’s celebration of the US National Park Service’s 100th Anniversary.

“I wasn’t invited back,” Brucker chuckled. But he didn’t learn his lesson either. 81 years later, he’s still addicted to exploring the world’s hidden spaces. He just chooses a more prestigious environment.

Mammoth Cave, the centrepiece of Kentucky’s Mammoth Cave National Park, has attracted explorers for centuries. Spelunkers have discovered blind fish in underground rivers, tunnels blooming with gypsum flowers and sheer, gaping pits. What they haven’t found is the end of the cave.

“It’s an obsession for a few people who are intrigued by how big it is, how far is goes and how many kinds of cave passages it contains,” Brucker said. “I think this is just unbridled curiosity fed by a continuous stream of discoveries.”

I think this is just unbridled curiosity fed by a continuous stream of discoveries

Brucker has worn a lot of hats – writer, executive, teacher, activist. But his favourite has a carbide lamp on the front. He’s been exploring the uncharted reaches of the Mammoth Cave System for more than 60 years. In that time, he’s played a role in most major discoveries, including Mammoth’s establishment as the longest cave in the world.

In the 1950s, Brucker was part of a team of ambitious young cavers exploring the Flint Ridge landscape around Mammoth Cave National Park. Together, they founded the Cave Research Foundation, a team of volunteers dedicated to scientific exploration. Their hope was to connect the local caves into a single giant system.

Up until then, mapping had been a disjointed and secretive process. In the 1840s, a slave and cave guide named Stephen Bishop published a sketch of Mammoth’s known tunnels. But modern explorers were less forthcoming. The available maps ranged from detailed illustrations to vague line diagrams. The Cave Research Foundation needed a sense of the whole.

Brucker decided to combine the tunnel surveys with a topographic map of the land above. That way, cavers could orient themselves vertically, horizontally and in relation to the surface.

“I spearheaded the mapping,” Brucker recalled. It took nearly two decades of persistent surveying to make the big breakthrough. In 1972, a Cave Research Foundation expedition connected Mammoth Cave to the surrounding Flint Ridge System. The discovery expanded Mammoth’s length to 144 miles, the longest cave in the world.

Forty-four years later, Mammoth has grown to 405 surveyed miles. That’s nearly twice the length of its nearest competitor, Mexico’s Sistema Sac Actun, and there’s no end in sight. Brucker and fellow caver James D Borden have projected it could stretch 1,000 miles or more.

Exploring a mile below ground isn’t the same as above. The names on Mammoth’s map evoke the obstacles cavers endure: Tight Spot, Fat Man’s Misery, Agony Avenue. Large-scale surveying is slow work involving hundreds of people. Even Brucker estimates he’s only covered 120 miles of the cave system himself.

But that’s far more than most will ever see. Only 12 miles of Mammoth Cave are open to the public, with groomed trails, lights and handrails. The rest belongs to spelunkers and scientists.

What does it take to be a caver? “Curiosity, the ability to see things in three dimensions and persistence,” Brucker listed. “I would not look for some kind of macho muscle man.” Not least of all because macho men can’t squeeze through narrow passageways. Size is often a deciding factor in caving: even the park’s Wild Cave Tour is limited to people measuring 42in or less at the shoulders and hips.

Outsized enthusiasts can still read their way through the cave. Brucker has co-authored four books on the exploration of Mammoth and published a historical novel about Stephen Bishop, the slave whose map was among the first published. He also spins tall tales for caving publications under his cheeky alter ego, Ergor Rubreck.

For Brucker, Mammoth remains as irresistible in retirement as the laundry chute did in kindergarten. He still participates in the Cave Research Foundation’s 10 annual expeditions (his wife, Lynn, is an expedition leader). His deep connection to Mammoth Cave’s past and present make him concerned about its future. The surrounding watersheds are leaky, and Brucker has campaigned to prevent industrial pollution from spilling into the park’s water basin.

“Some people see value only in terms of exploitation, but I think in terms of enjoyment by people,” he explained. “People you and I have never met and will never meet because they’re living in the next century. What will they find?”

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