Exploring Minnesota’s natural underground wonders: caves

There are miles of underground passageways in southeastern Minnesota that tell a story of the area long before it was a state.

Etched into the limestone of the 400 caves in that region of the state are fossils of creatures from 450 million years ago, when Minnesota was covered by an inland sea.

MPR News host Angela Davis talked about what makes these underground worlds wondrous with Aaron Bishop, the manager of Niagara Cave in Harmony, Minn.; Ian Pringle, an interpretive naturalist with the Minnesota Department of Natural Resources who leads tours at the Mystery Cave near Forestville, Minn.; and John Ackerman, the founder of the Minnesota Cave Preserve and the Minnesota Caving Club.

Here are five questions and a list of fun facts you should know if you’re a cave lover.

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The following transcript has been edited for length and clarity. Use the audio player above to listen to the full conversation.

Why is Southeast Minnesota the place for caves to be?

Bishop: We’re in a region called the Karst country. Karst is a type of topography that is characterized by caves or springs or disappearing streams, sinkholes and things like that. These are all made possible because around 450 million years ago, during the late Ordovician period, this entire region was covered by a very shallow saltwater sea and the carbonate mud that was deposited at the bottom of that shallow sea would eventually become the limestone bedrock that we get to delve into when we go into our caves and that’s why we can find seashells or fossils of creatures that once lived in the shallow sea.

Limestone is fairly easily dissolvable. Geologically, it’s sort of like the salt or the sugar of rock. It’s not as easily dissolvable as that, of course, but over hundreds of thousands of years, as water seeps through limestone cracks and vertical fractures or horizontal bedding planes, it can widen out those cracks and basically develop caves, either from running water physically eroding its way through or from chemical dissolution, kind of like if you have pop in your mouth for too long, it can start to dissolve your calcium carbonate teeth, very similar process. It’s just over a lot longer time span.

First hallway in Niagara Cave.
Courtesy of Niagara Cave

Is it possible to own a cave?

Bishop: From my understanding in the state of Minnesota, if you own land on the surface, you own it all the way to the center of the Earth, barring other previous agreements. So if you own land on the surface and you find a cave underneath on your own property, then that cave happens to be yours.

How do people find caves?

Bishop: There are depressions on the surface. In our case in southeastern Minnesota, we’ve got a lot of sinkholes. Not every single one leads into a cave for sure but some of them do such as a Niagara Cave. Some people find caves through horizontal entrances, I believe, like Mystery Cave. When that cave was discovered, my understanding is there was melted snow or a warm space because it was found in the wintertime. When it’s 48 degrees in the cave it melts the snow around there, and there’s an opening into that sidewall there.

A dripstone formation in Mystery Cave.
Danelle Cloutier | MPR News

Is it safe to visit Mystery Cave?

Pringle: One thing we are always concerned about is the safety of our guests and the scenic route is a great tour that leads out of the main entrance, out of the main visitor center here. It’s a very accessible route, you can actually get wheelchairs, strollers through there just fine. It’s not fully ADA-approved but we have it ramped.

We’re always telling people to watch their headspace, watch their footing because, you know, it’s 96 percent humidity down there, you walk on steel grates, they can be slick.

This cave actually has two different entrances to it. The one I just described is the scenic tour and then separated eight miles underground, you can reach the other entrance of Mystery Cave. It’s about two miles as the crow flies on the surface. But that entrance is a much more rustic experience, you’re going to be on compacted dirt floors, it’s uneven terrain. And with that, we always encourage and actually, it’s mandated that you go down there with close-toed shoes, good traction. And then we always recommend on whatever tour you’re gonna be on a nice sweater or something because of that 48 degrees.

Wishing well in Niagara Cave — The wishing well in Niagara Cave.
Courtesy of Niagara Cave

Have you noticed any changes in caves because of environmental changes?

Bishop: Probably the biggest changes we’ve seen would be the impacts from heavy rainfall, heavy and targeted rainfall. 2018 in Harmony, it broke the state record for most rainfall in a single year at I think 60.2 inches of rain, and we felt every inch of rain here at the cave.

We can get water in the cave. It’s not dangerous. It’s very slow when water rises in the cave, but the number of times that we’ve had to close off certain portions of the lower rooms because of too much water coming in, again, very slowly, it’s not an issue in terms of safety so much, it just fills the room and backs up into those last and lowest rooms of the cave, so they’re not accessible.

Mystery Cave's Turquoise Lake.
Danelle Cloutier | MPR News

Eight facts you might find interesting (or fun):

Soda straw stalactites at Niagara Cave — Soda Straw Stalactites at Niagara Cave.
Courtesy of Niagara Cave

1) Stalactites and stalagmites are formed by carbonic acid, the same component that makes a soda fizz.

2) Mystery Cave gets its name from the anecdote of a man that worked at the cave in the late 30s: “He was doing work down there and he had a wheelbarrow full of gravel and as he was pulling or pushing his wheelbarrow, it tipped over and the gravel spilled everywhere. And he said: ‘It’s a mystery anybody would want to work down here,“ Pringle said.