In our summer series "American Wonders," "CBS This Morning" is exploring places that make America wonderful, from majestic natural landscapes to spectacular creations. This week, Chip Reid takes you to a Tennessee cave for a tour of the largest underground lake in America. It attracts about 2,000 visitors a day from all over the world and is called "the Lost Sea."

Deep in the foothills of the Smoky Mountains, about 50 miles south of Knoxville, sits an American wonder 140 feet below ground.

Inside, a narrow tunnel leads to an intricate cave system drenched in history. Cherokee Indians used it as a shelter in the 1820s. And during the Civil War, Confederate soldiers used the cave's minerals to make gunpowder.

Tour guide Savannah Dalton is part of that rich history. Her grandmother and great aunt played down here as children.

"I was actually six years old the first time I came down here with my grandmother and older brother," Dalton said.

The cave system leads to the main tourist attraction: the lake.

"The lake was discovered by Ben Sands," Dalton said. "A 13-year-old boy who had actually crawled through a tunnel that was the size of a bicycle tire for 40 feet before he dropped down into the lake itself and actually waded out into about knee-deep water. It was a lot smaller when he came through. But we've blasted it out since."

Because of his discovery, the Lost Sea Adventure now sees about 150,000 tourists a year from all over the world.

A boat ferries visitors across the clear water where you can dip your fingers in the cold lake and watch rainbow trout swim by. Dalton said there are almost 300 rainbow trout in the lake.

"So they're down here long enough that they do lose a small portion of both their eyesight and their color," Dalton said.

The water is at least 70 feet deep and covers 4 and a half acres. Dalton said some of the cave's characteristics have been forming for probably hundreds, if not thousands of years.

Dalton said she enjoys her job just as much today as when she started more than seven years ago.

"There's a rich history here," Dalton said. "And it's something that you don't get to see every day. So just seeing the awe and the wonder on people's faces, when they come in and see this for the first time, it's-- it's pretty amazing."

So if you find yourself in the general vicinity of eastern Tennessee, you might want to stop by and check out this American wonder.