Every year, as many as 30,000 visitors to southeast Minnesota turn down a country road near Harmony, pull into a wooded lot between farm fields … and drop into a cave.

The big picture: Niagara Cave — named for a 60-foot waterfall thundering inside — is one of a half-dozen tourist-friendly passages into the underworld within a day's drive of the Twin Cities.

What to expect: Visitors descend 200 feet below the surface and wind through a narrow, mile-long main passage on an hour-long tour that features:

• 450-million-year-old fossils of ancient giant snails and squids.
• Ceilings that rise 100 feet in the cave's Grand Canyon area.
• Stalactites, stalagmites and formations that look like oozing slime — but are, in fact, solid rock.

"Awesome. It's something you definitely don't see every day," visitor A.J. McCormick told Axios.

• McCormick was visiting from southern Ohio during a cross-country road trip with his wife and three young kids.
• Inspired by a childhood trip to another cave, McCormick found Niagara Cave on a travel app and thought his kids would enjoy it: "I'd seen some of the pictures and I was like, 'This is really, really cool.'"

The latest: This year, the cave's owners are marking the 100th anniversary of its discovery by three local boys in 1924.

• They stumbled upon it while searching for a farmer's lost pigs.

Fun fact: Since opening for public tours in 1934, the cave has hosted more than 400 weddings in a small "chapel," Axios learned from Ryan Bishop, whose family owns and maintains Niagara Cave.

If you go: Regular tours cost $20.95 for adults and $12.95 for children older than 3. Prepare to descend — and climb back up — 255 steps.

• There are handrails and level floors of packed-down gravel throughout, but also plenty of chances to hit your head, so stick close to your guide.
• Guides will let you experience total darkness by briefly turning out the lights. If you wave your hand in front of your face and think you see something, that's your mind playing tricks on you.
• The cave is naturally 48 degrees year-round, so bring a light jacket.

Niagara is in Minnesota's caving hotspot — Fillmore County — which is home to more documented caverns than any other county in the state.

Between the lines: For millions of years, glaciers flattened much of Minnesota, burying much of the limestone where caves form beneath a layer of what geologists call "glacial drift."

Yes, but: These glaciers missed southeast Minnesota, so "the limestone was not buried," explained Bishop, who majored in geology in college.

• "Because the limestone is exposed to the surface, we have things like sinkholes and caves that are accessible to people."

Zoom out: This lack of glacial drift is why this region of Minnesota, Wisconsin and Iowa — known for its scenic, rolling topography — is called the Driftless Area.

The Driftless Area's geology is why cave tours can be enjoyed within a few hours' drive from the Twin Cities. Some are doable as day trips:

• Mystery Cave, located in a state park about 20 miles from Niagara Cave, is also open to the public. It's Minnesota's longest cave at more than 12 miles. (The cave only recently reopened for tours after flooding.)
• Crystal Cave is closer to the metro in western Wisconsin.
• Eagle Cave, southeast of La Crosse, Wisconsin, is a destination for youth groups who sleep overnight inside the cave, but it also offers basic tours on weekends.
• Spook Cave in northeast Iowa offers camping and cabins in addition to cave tours.
• Kickapoo Caverns near Prairie du Chien, Wisconsin, was recently restored by the Mississippi Valley Conservancy and offers occasional but irregular tours.

To get underground closer to home, check out St. Paul's Wabasha Street Caves or try navigating a tricky hiking path to the caves at Sorin's Bluff in Red Wing.

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Kyle's thought bubble: I made a day trip of my cave outing and stopped for lunch in Lanesboro, a picturesque town in a scenic valley along the Root River.

• Its streets are lined with cafés, shops and a brewery.
• Tree-covered bluffs rise in every direction, which makes it seem the town is at the bottom of a bowl.
• Lanesboro is also a bicycling destination along the Root River State Trail.

Go deeper: This landscape inspired locals to begin work on a long-distance hiking trail through the Driftless Area.