In just a few months, WYSO Curious has gotten four questions about the same topic: that bright blue lake off of Route 4 in Dayton.

John Todd of Fairborn was the first to write in to our site:

I have lived in the Dayton area since 1990, and I have ALWAYS wanted to know the story of that strange blue lake that can be seen from Route 4 and Stanley Ave exit. It is so strange, so beautiful, and seemingly out of place. I have heard many explanations of what this place is, but once and for all, I'd like to know the truth.

We also heard from Jay Krumholtz of Medway, Todd Wright of Tipp City (who sent a link to the satellite image of the lake—thanks!), and Marv Most of Sugarcreek Township, who asked why the lake is located right off the highway: “Normally you put things like that off a main road,” he said.

The bright blue lake can't be missed on Route 4 going in or out of Dayton from Wright-Patterson Air Force Base. It’s right next to the Mad River and multiple other man-made bodies of water, but it’s a completely different color: a weirdly brightblue-green, like a swimming pool.

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The color is so out of place in the middle of Dayton, says John Todd, “My youngest, Frankie, he’s four, he thinks it’s Florida. Like when we drive by he’s like, ‘there’s Florida!’”

But John’s always been bothered by the fact that “Florida” is fenced in, with no sign on the gate, and he’s heard all kinds of theories in his 24 years in Dayton.

“My friend’s step-dad was a chemist and he jokingly one night at a party said, that’d be a good place to put a body if you’re trying to get rid of a body,” he said.

So, we took a dive into the mystery: first, we found out what the blue lagoon is, and why it’s blue. Then, we got more background on whether it’s dangerous, and why it’s located where it’s located. Along the way, we learned some surprising things about Dayton’s unusual water treatment system, the color(s) of water, and the links between the blue lagoon and the Empire State Building.

What is that thing?

It might seem shrouded in secrecy from the fenced in look, but it just took a couple calls to find out the blue lake is part of the city of Dayton’s water treatment system, through which we found Phil Van Atta, the enthusiastic head of water supply and treatment for Dayton.

“It looks like a blue lagoon!” he said, as we drove the circumference of the lake in the snow.

When Van Atta looks at the blue lagoon, he doesn’t see anything sinister. He sees limestone, which is what’s in Dayton’s hard, mineral-filled well water. Hard water’s not dangerous to drink, but it can cake up pipes and water heaters, and make it harder to wash clothes or lather up soap. The city supplies softened water to Dayton, and many of the north and south suburbs—65 million gallons per day, which means a major water-softening operation.

Lewis Wallace / WYSO

Dayton softens its water using quicklime, this whitish stuff that’s another form of limestone; essentially, the quicklime bonds with the limestone minerals in the water, making it easier to filter out the hardness. What’s leftover after the softening process is a limey sludge, which most places that soften water store in lagoons until they can either ship it to landfills, or to contractors who use the lime as a fertilizer supplement on farms.

In essence, the blue-green lagoon is a lime sludge storage pond. The dredging boat and workers who sometimes appear out on the water work for a contractor called Synegro, which takes out the lime to spray onto farm fields.

But Dayton’s water system is actually different than most: the majority of the sludge is recycled back into quicklime.

“We have a sustainable practice,” Van Atta said. Dayton’s lime recycling plant, which was built in 1956, isn’t far from the Route 4 lagoon; they pump most of the residuals from the water treatment plants to the lime plant, change the chemical composition slightly, and then dry out quicklime in a giant kiln. They end up with pebbles of the water softening agent that get shipped back to the water plants by truck.

Sometimes the recycling plant has to close for maintenance, and that’s where the blue lagoon comes in: for just a few weeks out of the year, the lagoon becomes the dumping ground for the sludge that normally gets recycled.

Overall, the city’s closed water softening system is a big money-saver.

“We make more quicklime than we use at our water plants. And we sell the excess calcium oxide or quicklime to other water treatment plants,” he said. Quicklime sells for $105 to $150 per ton, and most municipalities purchase quicklime for water softening; the only other municipal water system Van Atta knows of that recycles lime is Miami-Dade in Florida.

The Dayton lime plant is in the midst of expanding its capacity in order to join with the cities of Troy and Middletown, who will send their lime sludge to the plant to be recycled. The $3 million update begins construction this year and should be complete by the end of 2015.

Lewis Wallace / WYSO

As for the question of why the lagoon is so close to the Route 4 freeway, the freeway didn’t actually go up until 1959, after the city started softening water and recycling the sludge. So, it’s likely that when the lagoon first went in it wasn’t quite as visible from the road. Van Atta also says it’s possible quicklime from the Dayton plant was used to build the new Route 4 in the 50s; lime can be used to stabilize mud when constructing a foundation for a road or building.

Why so blue?

The blue lagoon is there to store lime sludge, but why is it that Caribbean color?

It’s fairly simple: calcium carbonate, the fine limestone powder in the water, reflects the sunlight back and intensifies the naturally blue shade of water.

"Very, very small white particles will also scatter blue light very efficiently."

“Water molecules scatter blue light back out of the water much more efficiently than they scatter any other color,” said James Yoder with the Woods Hole Oceanographic Institute. Translation: water is blue.

“Very, very small white particles will also scatter blue light very efficiently, and that might even cause an intensification of the blue color or slightly change it to a different shade of blue,” he added. Translation: pale sediments like limestone or discarded shells make water look extra, super blue.

Basically, that aquamarine on a postcard from Florida is probably blue for the same reason as the Dayton sludge pond.

Is it dangerous or toxic?

The substance in the lagoon is just extremely hard water, which can dry out your skin, but isn’t likely to burn it off. But as for hanging out and sunbathing next to the blue lagoon, or taking a dive once in a while, Phil Van Atta says, please don’t. The water’s not toxic, but you could easily get stuck in the lime sludge.

“It’s not really conducive to that sort of thing,” he said. “It’s not the best place to hang out, you know. I would recommend going to a state park.”

If you’re really eager to try out hard-water diving, just check out southern Indiana, the limestone mining capital of the world. The area around Bloomington offers limestone tours with views of the state’s many abandoned quarries, which are varying shades of deep blue and aquamarine reminiscent of Dayton’s blue lagoon.

“It’s still a very thriving industry here locally,” said Julie Warren with Visit Bloomington. The Empire State building was largely built with Indiana limestone, as was the National Cathedral and parts of the Pentagon.

And there’s a bit of a tradition in Indiana: groups of young people go out to the quarries and cliff-jump, not that anyone’s recommending it.

“It’s totally illegal, you’re not supposed to do it,” Warren said, but just check it out on YouTube or in the 1970s movie Breaking Away. The danger, again, isn’t the water quality but the possibility of hitting your head on a rock or drowning.

So, save the beach towels for that trip to Florida—but enjoy the glow of the blue lagoon.

WYSO Curious is our growing, changing series driven by your questions and curiosities about the Miami Valley. Is there something you’ve always wondered about the Miami Valley’s history, people, culture, economy, politics, or environment? Send in a question now, and check back to see which questions we’re considering.

WYSO Curious is a partner of WBEZ's Curious City,which was founded by Jennifer Brandel and is one of ten Localore productions brought to life by AIR.