This Under-The-Radar Illinois Quarry Hides A Crystal-Clear Swimming Hole Worth The Short Hike In

Tucked into the woods of far southern Illinois, near the tiny town of Belknap, sits a former rock quarry that turned into one of the clearest swimming and diving spots in the state. Mermet Springs looks like a secret pond hidden among the trees, but underneath the surface it holds sunken planes, trains, and cars waiting to be explored.

Whether you want to learn how to scuba dive, cool off on a hot day, or just marvel at how blue the water gets, this quiet spot delivers way more than its low-key setting suggests. Best of all, it feels like a slice of the tropics without the plane ticket.

A Hidden Spring Off US-45 in Belknap

Drive down US-45 near Belknap and you would never guess that a world-class dive spot is waiting just off the road. Mermet Springs sits at 6724 N, US-45, in the far southern tip of Illinois, wrapped in trees and easy to miss if you blink.

The place is officially a diving center, and it has earned a remarkable 4.9-star rating across 188 reviews. That kind of score usually belongs to places people travel hours to reach, not a quarry tucked into rural Illinois.

What makes it special is the payoff versus the setting. From the outside it looks like a modest wooded lake, but people who make the trip come away calling it a hidden gem in the woods.

One visitor who stopped mid-road-trip described stumbling onto a beautiful, secret body of water they never expected to find out here.

From Working Quarry to Beloved Swimming Hole

Long before scuba tanks and dive flags showed up, this water was simply part of the neighborhood. One longtime local shared that their family lived right beside it before it became Mermet Springs, and they swam and fished there every single day.

That homegrown history is part of the charm. What started as an ordinary spot to cool off and cast a line eventually transformed into a destination that draws divers from across the region.

The transformation didn’t erase the friendly, small-community feel. The owner, Glen, is known for being on site regularly, and reviewers often mention how the whole crew treats guests like family rather than customers.

It’s a reminder that some of the best Illinois outings grew out of humble beginnings. A quiet quarry became a place where teenagers earn scuba certifications and families plan yearly trips.

Water So Clear You Can See the Surprises Below

Clarity is the headline here, and it’s no accident. Reviewers point out that the staff work hard to keep the water as clear as possible for divers, and one four-time visitor said the visibility was so good it showed up plainly in their underwater photos.

That crystal-clear quality is exactly what earns the quarry its reputation. When the sun hits it right, the water takes on a striking blue that feels closer to a tropical lagoon than a southern Illinois pond.

Good visibility matters because there is genuinely a lot to look at down there. Clear water means you actually get to enjoy the sunken attractions instead of squinting through murk.

Regulars describe the visibility as reliably solid trip after trip. For anyone deciding whether the drive is worth it, that consistency is a big part of the answer.

Trains, Planes, and Automobiles Beneath the Surface

Here’s the part that makes people’s jaws drop: the bottom of this lake is basically an underwater museum. One diver summed it up perfectly, saying you can spot trains, planes, and automobiles down there, literally.

These sunken objects give every dive a treasure-hunt feel. Instead of just floating around, you get to explore real attractions scattered across the quarry floor.

Visitors consistently rave about the underwater sights, which come up again and again in reviews. A group of divers mentioned getting to see amazing objects during their trip, and another called the diving here truly one of a kind.

It turns a swim into a story worth retelling. Not many Illinois day trips let you say you explored a submerged airplane before lunch.

Scuba Certification for First-Timers and Pros Alike

Never strapped on a tank before? No problem.

Mermet Springs is a go-to spot for learning to dive, with scuba certification courses and diving lessons landing among the most-mentioned topics in reviews.

The range of training is a big draw. People come here to earn various levels of certification, from complete beginners to divers building experience, and rental equipment is available if you don’t own gear yet.

The instructors get glowing praise. One parent watched their son earn his certification and called the instructor awesome and the owner unbeatable, while another reviewer, after trying several dive schools, ranked these instructors the best they’ve had.

There’s real heart behind the teaching too. A St. Louis teacher shared that 13 of their high school students became certified divers here, many earning their Open Water card before they even got a driver’s license.

Friendly Dive Guides Who Know Every Corner

Show up without a dive buddy and you won’t be stuck. One traveler who stopped mid-road-trip took advantage of one of the on-site dive guides and came away thrilled, describing the guide as extremely knowledgeable about the spring and its attractions.

That expertise runs through the whole team. Reviewers repeatedly call the staff friendly, patient, and helpful with any question you might have, and several note that the front desk is the place to start when you arrive.

Nervous newcomers seem especially well cared for. A hesitant first-timer said her instructors kept her feeling safe and calm the entire time, and once underwater she got to feed the fish and see some genuinely cool things.

People clearly feel welcomed, not rushed. That warmth is a recurring theme in review after review.

Night Dives, Deep Dives, and Underwater Music

One of the quirkiest touches here happens when you descend: there’s underwater music playing below the surface. A reviewer said it helps call you down, and it comes up as one of the more memorable features of a dive.

Beyond that novelty, the quarry offers real variety. Night dives and deep dives both show up in the popular topics, so experienced divers have plenty of ways to keep things interesting.

Depth adds to the adventure. Divers mention that the best sights sit around 45 feet, giving you a clear target zone packed with things to explore.

Between the tunes, the darkness of a night dive, and the deeper attractions, no two trips have to feel the same. It’s the kind of place that rewards coming back more than once.

Cool Water and a Reminder to Pack the Right Gear

Fair warning for the sun-lovers among us: this water can get chilly the deeper you go. Water temperature is a topic divers bring up often, and it’s worth planning around.

Several reviewers give the friendly heads-up that the water turns cooler past about 50 feet, which is normal for a spring-fed quarry. One visitor laughed that they needed a dry suit because it was a bit too cold for them, even while calling the place awesome.

The takeaway is simple. Bring a wetsuit or dry suit if you plan to go deep, and you’ll stay comfortable enough to enjoy every minute.

Up near the surface, where a lot of the fun stuff sits around 45 feet, the temperature is far more forgiving. A little preparation goes a long way toward a great day.

Clean Pavilions, Camping, and Family-Friendly Fun

The setup on land is as welcoming as the water. Reviewers consistently describe the facility as very nice, clean, and well kept, with tables to organize your gear and a layout that feels easy to use.

Families fit right in. One parent takes an annual group trip here and said the diving was great for the kids, with lots of fun things for them to do in the water, and they’d already booked their return before leaving.

You can even stay overnight. Camping is available with nice level spots to set up an RV, though one reviewer notes there is no water hookup on the property, so plan accordingly.

Little touches build trust too. A longtime returning diver appreciated that gear stays put and things don’t get stolen there, which makes relaxing a whole lot easier.

Getting There and What to Know Before You Go

Planning the trip is refreshingly simple. Mermet Springs is located at 6724 N, US-45, Belknap, IL 62908, sitting in the far southern part of the state where the woods start to feel wonderfully remote.

Before you load up the car, it’s smart to reach out first. You can call ahead at +1 618-527-3483 or check the details at mermetsprings.com, since reviewers mention flexible scheduling as one of the site’s perks.

Once you arrive, head straight to the front desk. Multiple visitors stress that checking in there first is the way to go, and the staff will happily answer any questions before you get in the water.

Even curious non-divers are welcome. One person just stopped by to learn more about diving and left impressed by the lake and the knowledgeable crew.

Why This Quiet Quarry Is Worth the Drive

If you can’t swing a trip to the Caribbean, this might be the next best thing. One reviewer put it exactly that way, calling Mermet Springs a great substitute for tropical diving right here in southern Illinois.

The value is hard to beat. You get clear water, sunken planes and trains, friendly guides, affordable rental gear, and camping, all wrapped in a setting that feels like a private discovery in the woods.

What lingers most is how people talk about the community. Divers describe feeling like part of a family and returning year after year, and that sense of belonging is tough to fake.

For Illinois families, curious first-timers, or anyone chasing an easy escape with an outsized payoff, this under-the-radar quarry earns its 4.9 stars. Pack a towel, point the car south, and see what’s hiding beneath that clear blue surface.