When training for triathlons at small health club pools, one big hassle is the large number of turns (as many as 80 to 100) it takes to swim a mile. But in the largest pool in the world, just one lap is enough to cover a mile and then some. That's because the world's largest pool — defined as a separate, manmade body of water with some form of circulation — is over a half mile long and contains some 66 million gallons of water.

The pool at San Alfonso del Mar, a private resort in Algarrobo, Chile, just west of the capital, Santiago, seems more like a pristine lake than something resembling a swimming pool. Even the resort brochure describes it as a "crystal clear saltwater lagoon."

But just like a pond or a vinyl-liner pool, the lagoon/pool is lined with a thick membrane. Only this membrane rolls out to cover an area of 870,000 square feet. Eventually, that liner will need to be replaced. No doubt pool companies the world over are already bidding on that lucrative liner replacement contract.