The next time you flush your toilet or take a drink of water, think of six burly deep-sea divers who spent a month together in a capsule the size of a small RV.

What’s the connection?

New York City’s main water-supply system sprung a 20-million-gallon-a-day leak, and to fix it, the divers had to take turns going down nearly 700 feet deep to twist wrenches, take measurements and operate heavy equipment.

In between shifts, they spent down time in a chamber that mimics the atmospheric pressure of the underwater abyss.

The crew spent 35 days in Dutchess County replacing a plug in an underwater shaft that connects the city’s water supply to an upstate reservoir system.

The plug installation was the first stage of an ambitious $2.1 billion initiative to drain the shaft, and build a bypass tunnel around a leaking section of the 85-mile Delaware Aqueduct, which supplies roughly half the city’s drinking water.

They lived in a sealed 24-foot tubular pressurized tank complete with showers, a television and a Nerf basketball hoop.

They left only to transfer to a two-man diving bell that was lowered into the earth, where they took turns working four-hour shifts.

The only way to avoid the fluctuating pressure in between shifts was to hunker down in the decompression chamber.

Not only did that mean living for five weeks in a bunker that makes a subway car seem like a two-bedroom apartment, it meant breathing in air made largely of helium and talking in a voice that sounds as if it’s being played on fast- forward.

“At this depth, it’s hard to understand sometimes,” said diver Travis Rudolph. “Your conversations are pretty limited. There’s a lot of ‘What?’ ”