Twenty million gallons a day. That's how much has been leaking from the Rondout-West Branch Tunnel originating in the Catskill Mountains and supplying much of New York City's water. Teams of divers live in a pressurized tank far below ground for a month at a time, working on repairing things, and breathing a helium-rich air mixture that makes them squeaky voiced.
Here's some more info on this deep lifestyle. Scroll down to hear the helium effects in song.