Minnesota man spends millions on mission to save hidden world

Miles of unexplored caves are buried deep below Minnesota, and one man is on a mission to find and buy as many as he can to save a hidden world.

On the surface, John Ackerman's cornfield in southeastern Minnesota looks pretty ordinary, but when you dig a little deeper, you find something remarkable.

It's called the Spring Valley Cave, the largest privately owned cave system in the state.

Ackerman is determined to protect the unique subterranean environment. He told CBS News' Michelle Miller he's discovered 45 caves so far over 40 miles of underground caverns.

Through his organization, Minnesota Cave Preserve, Ackerman has been buying up caves and the land over them since 1989, spending what he guesses is around $4 million.

Ackerman said, "My biggest thrill is being the first one to introduce my light into inky blackness. Each cave system is unique into itself. Some of them have raging rivers, waterfalls, immense rooms."

It's not just the unknown or the unexplored. When Ackerman ventures into a cave for the first time, it's in complete darkness.

You'd think something made of solid rock would be indestructible, but Ackerman says the caves are actually under constant threat from above - from things like fracking, pollution or commercialization.

He said, "You can't protect what you don't know. And so it's so critical to have these caves protected and studied."

For scientists, Ackerman says the history etched in stone holds clues to the Earth's evolution - what the weather and wildlife were like before recorded history.

Asked why that's important, Ackerman said, "We've pretty much plundered our natural resources above ground. These are incredibly rare and need to be protected so that scientists and others can study these caverns long into the future."

And when he talks about caves carved out over eons, Ackerman takes a long view. In one of his caves, he said, "I took legal steps to establish a formal cemetery above this very room. I have also gotten permission from the state of Minnesota to be buried in this room. I'd like to see them attempt to commercialize this cave and run tours over my dead body."