Mystery Cave believed to be the largest in Minnesota with 13 miles of passages, underground lakes

FILLMORE COUNTY, Minn. — The Fillmore County countryside contains thousands of acres of green, lush farmland, but some people here believe what's below the surface is just as beautiful.

"There are actually over 400 caves in this county. This just happens to be the largest of the caves," said Greg Anderson, Assistant Cave Manager.

It was discovered by a man named Joe Pettey in 1937 and he may very well have been the first human to set foot inside. It got its name years later when a man named Clarence accidentally spilled a wheelbarrow full of debris while working at the cave.

"And he turned to his co-workers and said it's a mystery as to why anybody would want to work in a cave all day," said Anderson.

That's how the name Mystery Cave was born. But there are reasons why Anderson enjoys working here all day.

"This is one of the great features in our cave known as Turquoise Lake," said Anderson.

Turquoise Lake's deepest point is 10 feet. Calcite crystals in the water give it its beautiful blue color.

Millions of years ago, the entire cave was underwater. Fossils are proof of that.

"It's called a cephalopod. It's an ancient relative of a squid or an octopus," said Anderson while pointing at a fossil in the rock.

But that's not even close to the oldest thing you'll find here.

The rock inside the cave is 450 million years old. To put into perspective how old that is, dinosaurs were here 65 million years ago.

It's dark and damp in these corridors, but even still, bats, insects and one other little creature call the cave home.

"If you look at the surface of this pool, you'll see what looks like little white specs of dust. Each one of those is alive," said Anderson.

They're called springtails and they feed off organic material in the water.

"They are blind, colorless, invertebrates and they can move by flipping their tails," said Anderson.

These days, visitors of all ages can visit a place that's defied age. Even after millions of years, there's always something new to discover.

"It was amazing. Great tour. Very cool to see. It's unbelievable that we are in Minnesota, and we have this cave down here," said Lucinda Rose, a visitor.

It's also estimated that Fillmore County has about 10,000 sinkholes.

On Aug. 9 and 10, Mystery Cave will hold a dark sky festival where astronomy clubs from across the state will gather with their telescopes. For more information, you can check out the cave's website .