CHEESMAN MOUNTAIN, Colo. -- A daring operation is under way, deep under water at Cheesman Reservoir.

Divers are attempting to replace the dam's aging gates which allow water to pass through the structure.

The current gates were installed in 1895, when the dam was built.

The plan for divers to replace the gates was thought to be better than the alternative, which would have required draining the lake. That’s something that hasn’t been done in more than a century.

Draining the lake could have also jeopardized Denver’s water supply and taken months to refill.

"Then we would be crossing our fingers and hoping for a good snowfall," said Jeff Martin, dam safety engineer for Denver Water.

Cheesman is a critical part of Denver’s water supply, and was the first reservoir to ever serve Denver, affording the city a more secure water supply, rather than relying on river water from mountain runoff.

This option will take ten weeks to complete.

A barge in the lake is now the home of what could be called the big descent.

"We’re replacing aging infrastructure within the dam," said Martin.

For the divers, it is a journey 200 feet to the bottom of what was once the world’s tallest dam.

"And it’s a very unique dam. There are not a lot of masonry dams in the country or even the world," said Martin.

Divers are installing new stainless steel gates which allow water to pass through the dam. Two teams of two divers working 12 hour shifts are in a pressurized chamber for 28 days straight.

"They’re in a chamber which I guess you could equate to a big propane tank," said Mark Hintz, dive superintendent with Global Diving and Salvage Inc.

The divers are supported on deck by a 20 person crew.

"They have a cook. Good food. You burn a lot of calories in there," said Hintz.

For the entire month the divers live in a chamber and breathe 90 percent helium and 10 percent oxygen, which makes them sound a little like Donald Duck.

"They have a motorcycle looking helmet, or space-like helmet. It’s probably an odd scene if you could see it from the outside," said Hintz.

If all goes well, the dam will be fine for another 100 years. Big pressure -- both literally and figuratively -- on the divers to restore the dam’s aging gates.

"The storage here is about 80,000 acre feet," said Martin.

"Everyone’s got to be safe or they’re not on the job," said Hintz.

The crews "pressed down" on Monday night.

They are in for 26 days and after they finish, divers will then be required to decompress for two days as oxygen is reintroduced to their bodies.

"We’re very proud of it. It’s still working and doing the same function it did in 1905," said Martin of the dam.

The dive teams ride a pressurized diving bell down to the bottom of the lake.

They work in special canvass wet suits that are heated with warm water. They then sleep and rest for 12 hours, still in a pressurized chamber, get up and do it all over again.