San Alfonso del Mar

San Alfonso del Mar is a private resort in Algarrobo, Chile, about 100 km (62 mi) west of the capital Santiago. The resort is recognised as having the world's largest swimming pool.
The pool is 1 km (0.62 mi) long, covering 8 ha (20 acres), containing some 250,000,000 litres (55,000,000 imp gal; 66,000,000 US gal) of seawater, with a maximum depth of 35 m (115 ft). It uses water pumped, filtered, and treated from the Pacific Ocean.
The pool was developed by Chilean company Crystal Lagoons, and opened in December 2006. While early estimates put the total cost of construction at about US$3.5 million for the filtration system alone, more recent estimates are in the area of US$1.5 billion to US$2 billion total for construction and almost US$4 million in annual maintenance.