San Alfonso del Mar
Located in the resort town of Algarrobo on Chile's southern coast, the San Alfonso del Mar has changed the way the world looks at swimming pools. That's because this pool is large enough to sail a boat in, and if you swim its entire length, you'd cover 8/10ths of a mile (1.3 km). Measuring more than 20 acres (8 hectares), the San Alfonso is 6,000 times larger than your average hotel pool and was named the largest pool in the world by the Guinness Book of World Records in 2008 [source: Crystal Lagoons].
The pool took five years to build and cost nearly $2 billion USD (about $1.1 trillion Chilean pesos) [source: Tatko-Peterson]. Developers used an innovative salt-water filtering technology created by Crystal Lagoons to purify more than 66 million gallons (2.5 million liters) of ocean water to fill the pool [source: Crystal Lagoons]. It opened in 2007 as part of the San Alfonso del Mar resort, which is home to thousands of apartments, condominiums and hotel rooms. While the pool isn't open to the public, all resort guests and residents have unlimited access.