At 1,000 yards long, this man-made crystal-clear salt-water lagoon covers 20 acres, has a 115 ft deep end and holds 66 million gallons of water. Located at the San Alfonso de Mar resort in Algarrobo, Chile, the pool took 5 years and 1 billion pounds to build and 2 million pounds a year just to maintain. It draws water from the sea and filters it, keeping it in constant circulation. Warmed by the sun, the pool reaches 26C or about 9 degrees warmer than the sea next to it. Via.