SPRING VALLEY, Minn. -- John Ackerman is a big-time landlord of subterranean real estate.

Ackerman, 59, owns Spring Valley Caverns, the largest private cave in Minnesota but just the beginning of his underground empire, which he calls the Minnesota Cave Preserve. He holds the keys to more than 65 kilometres of caves hidden beneath the rolling farm fields of Minnesota and Iowa and is always seeking more.

"I think it's just to be able to be the first human being to introduce light into the inky blackness of just unknown chambers that may go for miles," he said of his motivation. "And then later it's to protect them so that they're available for scientific research. That's what keeps me going. That's the adrenaline rush, is 'What's around the next corner?"'

Ackerman estimates he's spent $4 million on cave exploration and acquiring underground rights, but he doesn't charge admission to the nature groups, scientists and cavers who visit. It's a hobby made possible by his successful furniture restoration business, he said.

Ackerman, who lives in Farmington, just south of Minneapolis, said he discovered his love for spelunking as a boy when he poked around the caves along the Mississippi River in St. Paul. Spring Valley Caverns was his first acquisition, in 1989, when he bought 240 hectares acres of farmland. A previous owner had tried and failed to commercialize the 800-metre-long cave below.

Southeastern Minnesota and northeastern Iowa are prime cave terrain, with soluble rock such as limestone eroded over time by running water. For a person on the surface, sinkholes are a clue to finding caves, said Ackerman, who claims to have discovered 43 caves that he now owns. All but one are in southeastern Minnesota.

Once he finds a cave, Ackerman says he approaches the landowner to buy some surface land with underground rights to the rest of the land. He then has an entrance drilled.

On a recent trip to Spring Valley Caverns, which covers about nine kilometres, Ackerman wore a light on his red helmet to pierce the darkness. Dripping water was the only sound, and bats, just beginning to move into the cave to hibernate, were the only animal in sight. It was chilly; Ackerman says the cave stays a consistent 9 degrees.

A remote section of the cave has a roaring river and a room big enough to hold a house, Ackerman said.

"It's really an underground paradise. It's unbelievable down there," he said.

Calvin Alexander, an earth sciences professor at the University of Minnesota, describes Ackerman as both a friend and "one of the most Type A personalities that I've run into in a long, long time."

"He has single-handedly made available to scientists more miles of cave passage in Minnesota than anyone by far," Alexander said.

His pursuit of caving hasn't always gone smoothly. Ackerman skirmished with the Minnesota Department of Natural Resources after the department bought a cave and erected a gate in 1999 to preserve it and keep out trespassers.

In 2004, Ackerman bought some adjoining surface land along with underground rights to part of that cave and drilled a 22-metre shaft to reopen it to cavers.

"That sort of thing rubbed people the wrong way," said Dean Wiseman, a spokesman for the National Speleological Society, a non-profit dedicated to exploring and conserving caves. "He chose a course of action that some people disagreed with."

Ed Quinn, a resource manager in the DNR's Division of Parks and Trails, said his division had no issues with Ackerman nor any plans to pursue any of his caves.

"We only acquire lands from willing sellers," Quinn said.

Exploring caves is dangerous work, Ackerman said.

"I've had it all happen. Rock falls, near drowning, being stuck, running out of lights," he said. "Again, what I do (is) inherently dangerous, but the rewards for me, that's the payoff."

Ackerman is divorced and has three grown children who aren't interested in taking over the caves. He has approached the Minnesota Land Trust about preserving them, something the organization is weighing, according to a spokesman.

"Caves are scarce," Ackerman said. "And it's my wish to make sure the scientific community has access to all these caverns when I'm long gone -- perpetual access."