Back in the late 1920s, a sinkhole fell open and gave way to something amazing: miles of cave tunnels in Corydon, Ind.
It's one of half a dozen caves in the country where discoveries are still being made, and there's more to explore.
Below the rolling hills and farmland in southern Indiana is an entirely different world.
"There's not a whole lot of frontiers left on the earth," says caver Dave Everton. "There's the ocean that's unexplored, maybe outer space, but caves are one of the few places a person can go that nobody has ever been, so that makes it kind of unique."
The Binkley Cave System in Corydon is a network of caves that extends for 42 miles. Its Indiana's largest cave system and much of it is unexplored.
Nearly 50 Miles Of Caves
"They think of Indiana as cornfield and soybeans, but this part of Indiana that wasn't glaciated is just like the Ozarks and as we say it has some of the biggest caves in the world," says Gary Robertson.
Robertson first crawled into a cave as an 11-year-old Boy Scout and took to exploring underground as soon as he could drive.
He's a big reason why people can now venture safely into a small section of Binkley at Indiana Caverns.
People have been going into caves for a long time, but Native Americans and early European settlers weren't so much interested in mapping the caverns as they were using them for shelter.
Amateur cavers navigate cramped, dark spaces to survey the elaborate cave systems.
Robertson and his group venture through total darkness, for hours at a time, sometimes doing a military crawl through tight passageways. They keep discovering more areas of the cave, and it keeps getting longer and longer.
It used to only be about 22 miles, but now it's 42. Robertson suspects there could easily be 100 miles in the cave system.
It already ranks among the longest in the country and it might be the longest cave system that's privately owned. A portion of it is open to the public, and making it so wasn't easy.
"It was really hard," Robertson says. "It took us 54 weeks to develop the cave. There were about six people doing all the work underground...tons and tons of steel, boats that went in on zip lines - a lot of blasting and stuff. Developing a cave is not something you hire a construction company for unless you're the government. Otherwise you gotta kinda do it on a shoe-string."
Still More To Be Explored
Boat rides let visitors venture through the winding underground passageways where prehistoric bears used to sleep and blind cave creatures have lived for thousands of years.
Paleontologists are still shuffling through the vast amounts of prehistoric skull and bone left in the cave and have discovered places where a herd of flat headed prehistoric boars got trapped and fought to get out, or a juvenile bison died.
That room of Indiana Caverns contains valuable information for these scientists, discovered by adventurers willing to dig a little deeper.
"We were the first people there," Everton says. "I can't take credit for that, but it just so happens we were there and that was a really neat thing to be a part of…and knowing there's a treasure trove of information for other people to come along and study."
They need a special permit to get behind the ropes at Indiana caverns and conduct their studies, but tourists can visit all year round, and whether it's July or December it's always a moderate 57 degrees down here.
It'll be up to the next generation to continue the work exploring the cave system. Robertson doesn't see an end to it in his lifetime.
"Some 11-year-old may crawl through the saber tooth cave and decide, 'Hey, I want to be a cave explorer,'.and they can have a career like I've had in the future."
