Custer County, South Dakota, is a place that few folks give much thought to; it's quiet, and for nature lovers, it's been a fan-favorite for a long time. It's a great place to enjoy the great outdoors and how - the state is astonishingly lovely, and this little segment is especially so. Enter Jewel Cave, an outrageously incredible natural wonder smack-dab in the middle (or underneath) of it all. This cave is the very definition of a hidden gem; sure, lots of South Dakotans know it's there, but folks from out of state tend to tragically overlook it. Oh well - all the more for us!

Now, here's what makes this cave special: it just so happens to be one of the longest caves in the world. Yes, the world - and the enormity of this is tough to wrap one's brain around. At the time of this writing, there are well over 220 miles of this cave that have already been mapped and explored - and still more of it waiting for the same. Some describe it as an "underground wilderness," which is delightful. It fits nicely.

The cave is open daily from 8:30 a.m. until 4:30 p.m., and it's so amazing that I strongly recommend a multi-day excursion. There's simply no way to explore the entirety of it in one trip, unless that trip is months long. Airflow studies have shown that much of the cave is still undiscovered, and considering how much we already know of, that's pretty incredible. It's estimated that there could be up to 7,000 more miles of undiscovered cave here.

Jewel Cave is unique in many ways and for more than just its enormity. Unlike other caves of similar size, this one was not formed thanks to underground rivers. Instead, it formed thanks to the circulation of highly acidic groundwater. Interestingly, the nailhead spar crystals you'll see all over the place formed when everything was still underwater. Jaw-dropping cave formations include awe-inspiring finds like stalactites, stalagmites, flowstone, draperies, and popcorn formations.

Take a tour of Jewel Cave to learn more - it's worth every single moment, even for junior spelunkers just dipping their toes into the science of caves.

This incredible cave in South Dakota is also ridiculously rich in human history. It was discovered in the early 1900s by a pair of brothers, who were unsuccessful in later attempts to mine the cave for crystals. Once that flopped, the boys pivoted to tourism, and this was a considerably better move.

In 1908, Jewel Cave was designated a National Monument by President Teddy Roosevelt, who famously had a soft spot for the Dakotas and subsequently had one of our most incredible natural parks named after him in return.

Here's the best news of all: there's no entry fee to come and enjoy this incredible natural wonder in South Dakota. All cave access is by guided tour only, so please be sure to reserve your place ahead of time on the official Jewel Cave National Monument page on Recreation.gov. Are you planning to visit this amazing hidden gem in South Dakota and need to plan your trip? Check out all the helpful options from our friends at Expedia Travel.

Do you have a favorite hidden gem in South Dakota? Tell me all about it in the comments.