No one paid much attention to the 400-foot-deep pit at the edge of the Nashville International Airport complex.
Not until the 2010 flood, says Christine Vitt, the airport's head of strategic planning and sustainability. That's when the abandoned limestone quarry overflowed for the first time ever.
"And we had the energy consultant there, who said, 'Well hey, maybe there's a win-win here,'" Vitt recalled.
Five years later, that thought has led to a plan airport officials hope will save as much as $400,000 a year.
The proposal — use some of the estimated 1.5 billion gallons of water trapped in the quarry to construct a giant, gurgling air conditioner.
Here's how it would work: Even in summer, when temperatures in Tennessee can exceed 90 degrees, the water remains just 50 degrees at a depth of 50 feet.
So Vitt's team has proposed pumping some of that cold water in the airport's HVAC system, where it can chill the air cheaply and efficiently. Other pipes would then circulate it back to the quarry in a continuous loop.
"It's just pumping the water into a closed piping system and recirculating it back and forth," Vitt said.
The idea is not new, but the project would be the largest geothermal system of its kind in the United States.
Officials say no other airport has such a system — mainly because they don't have an unused, 43-acre pool of water sitting nearby.
The quarry has been around for decades. It sits at the far edge of the airport complex — down the long taxiway across Donelson Pike.
As the airport has expanded, officials have left the quarry alone to catch runoff. The water is undrinkable, but it can be used for cooling and other purposes, Vitt said.
In fact, the airport also may siphon off even more water from the quarry to irrigate its plants and trees.
Airport officials gave initial approval to the project Wednesday. It would cost about $10.4 million, making it years before the savings pay off.
But officials hope to secure state funding, as well as a green-energy grant from the Federal Aviation Administration, to help defray the cost.