SPRING VALLEY(AP) - A Minnesota man is a big-time owner of underground real estate.

Fifty-nine-year-old John Ackerman owns Spring Valley Caverns, the largest private cave in Minnesota.

That's just the beginning of Ackerman's subterranean empire. He holds the keys to more than 40 miles of caves hidden beneath the farm fields of Minnesota and Iowa.

Ackerman says he got the itch to explore caves 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 600 acres of farmland.

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.