Hold your breath and submerge your face in cold water and your heart will immediately slow down by as much as 25 per cent. This is known as the “diving reflex” and it’s something we share with other mammals. It means we can survive longer without oxygen submerged in cold water than we can on dry land. Although it is an instinctive reaction, it can also be trained: free divers (the sport of deep diving without any breathing apparatus) practice the art of meditation and deep breathing in order to slow their heartbeat.
The deepest free dive ever achieved (using fins) is 896ft (273m), reached by Croatian diver Goran Colak in Lignano, Italy, in 2011. The record for holding one’s breath under water (static apnoea) is held by Stéphane Mifsud, a French free diver who managed 11 minutes 35 seconds in 2009.
The record for the deepest any human has ever dived was set in 1960 by Jacques Piccard, and his assistant Don Walsh, in an area of the ocean called Challenger Deep, part of the Mariana Trench in the Pacific. The trench is 1,580 miles (2,550km) long but only 225ft (69m) wide, and reaches a depth of about 6.8 miles (11km). It took them four hours and 48 minutes travelling in Trieste, a pressurised bathyscaphe (Greek bathos, “deep” + skaphos, “vessel”) in 1960. According to Piccard, “the bottom appeared light and clear”. In reality, the pressure there is so great you would have to heat water to 530C (986F) to get it to boil. In 2012, the film director James Cameron followed in their footsteps, and reached roughly the same depth in around 70 minutes.
The life aquatic
Aquatic mammals have used the reflex to allow incredible feats of underwater endurance. The sperm whale can dive the deepest of any air-breathing animal: they’ve been identified more than a mile (2km) down for around two hours at a time, where they hunt for squid. They expel all the air from their lungs to avoid the risk of “the bends” and survive on the oxygen absorbed in their blood. They can dive the length of a football pitch every minute.
Elephant seals can dive for two hours at a time, and reach depths of 5,000ft. Their bodies hold twice as much blood as most mammals and, when they’re diving, their heart rate plummets from 90 to just four beats a minute. To help them sink faster, some will even swallow stones.
A seal’s eyes don’t go blurry underwater. In other mammals, this blur is caused by the outer lens (cornea) being rendered useless by the water, like a transparent glass marble which disappears when you drop it in the bath. Seals overcome this through a huge spherical inner lens to focus the image, and an extremely adjustable iris to control the light. This not only gives them their big-eyed charm, it also means they can hunt in bright sunlight as well as the gloomy ocean depths.
Gannets have no nostrils. Because they dive headfirst into the sea – hitting the water at 60mph – and need to keep water out of their lungs, the birds breathe solely through their mouths. They can dive down to 25m (82ft) below sea level. Geese lose altitude by “whiffling”; they nosedive, spiralling as they go, sometimes flying upside down with their heads pointing straight ahead.
In the St Louis Olympics of 1904 there was a very odd event called the “plunge for distance”. Competitors dived into a very deep pool and remained motionless until they bobbed up again. The winner was the one who sunk the farthest without attempting to propel himself through the water.
The gold medallist was an American – 20-year-old William Dickey – with a plunge of 62.5ft (19.05m).
“Scuba” is an acronym for self-contained underwater breathing apparatus. In North Carolina, there is an annual underwater bike race undertaken by people in scuba gear. In the Maldives there is an underwater spa, where passing fish observe your massage; there’s also an underwater restaurant 16ft (5m) below sea level.
But the hotbed of underwater activity is Florida: the Lower Keys Chamber of Commerce stages an annual underwater concert which gig-goers can attend in their scuba gear; or you can get married underwater with a live video feed to guests on the surface