Enough to make your head swim: World's largest pool which cost $1bn and holds 66m gallons is so big you can even sail boats on it

The Crystal Lagoon at San Alfonso del Mar resort, Chile, is large enough for sailing and comes with its own fake beach

By Lyle Brennan

At the San Alfonso del Mar resort in Chile, a quick dip could well turn into a marathon.

Swimming a length in this, the world's largest outdoor pool, would mean stroke after stroke for more than three fifths of a mile - that's 20 Olympic-size swimming pools.

The enormous man-made lagoon is set halfway up the country's Pacific coast, in the city of Algarrobo, and is filled with 66 million gallons of crystal clear seawater.

A little slice of the Pacific: The world's biggest pool is three fifths of a mile long and draws water from the ocean, cleaning it and allowing the sun to warm it to 26C

It also hold the Guinness record for the world's deepest - so if you don't feel like diving 115ft to the bottom, it might be best to bring some spare goggles.

The pool opened in December 2006 after nearly five years of construction work and is said to have set developers back as much as £1billion.

And on top of that eye-watering initial cost, it takes a further £2million a year to keep it in working order.

It uses a computer-controlled suction and filtration system to suck water in from the ocean at one end and pump it out at the other, while the sun warms it to 26C - nine degrees higher than the sea.

The pool's incredible dimensions leave the next biggest floundering in its wake, with the Orthlieb in Morocco measuring a mere 1,575ft long.

Lost at sea: A bather floats on the amazingly clear waters of the vast Crystal Lagoon, at the San Alfonso del Mar resort, Chile

Dazzling blue: A small boat sails round a platform jutting into the lagoon's 20-acre expanse

Big drip: The pool, shown left next to the city of Algarrobo, is thought to have cost £1billion to build and runs up £2million a year in maintenance

Man-made paradise: A computer-generated image of the San Alfonso del Mar resort shows how its semi-circular artificial beaches and filtered waters sit right next to the real thing