Let me just say it: I hate treadmills. Running on them is the highest form of drudgery. Put Sisyphus on one and even he'd say, "Oh, to hell with this."

Don't get me wrong, I love running. In the last three years alone, I've logged more than 2,000 miles (in heat waves, blizzards, rainstorms—even after some minor surgery). The point is, I'm committed. Just don't ask me to run in place.

And then I heard about it: the HydroWorx 2000, the ne plus ultra of underwater treadmills. This 5,750-gallon, six-foot-deep pool comes with an adjustable moving floor, underwater cameras, massage hoses, and a $167,900 base price tag (a home version starts at $16,400)—not bad for the training weapon of the Navy SEALs and many pro sports teams. Alberto Salazar, the legendary marathoner and head of the Oregon Project, Nike's high-tech running program, has made HydroWorx pools a cornerstone of his regimen. The logic is simple: In water, a runner weighs less than he would on land. Reduced weight means reduced stress on joints.

"Nothing replaces running outside," says Salazar, "but this becomes the best way to get in that extra mileage."

So underwater running may be the salvation for elite athletes, but how about someone whose marathons are measured on, say, a sundial?