The interwebs are awash today with new footage of the past world-record holding, free-diver Guillaume Néry plunging to the of the world’s deepest swimming pool. The NEMO 33 in Brussels, Belgium is unsurprisingly 33 metres deep and contains an unfathomable 2.5 million litres of non-chlorinated spring water. It’s mostly used to train divers and even has simulated caves at 10 meters.
So while the video is visually impressive, and the soundtrack (Portishead’s “Deep water”) straddles cute and haunting, it doesn’t come close to capturing just how far humans can dive. The current constant weight free diving world record is held by Herbert Nitsch who dove to an incredible depth of 124 meters in 2010 . Nitsch also holds the no-limits free-diving record where divers can use a weighted sled to help with decent and an air-filled balloon to help them return to the surface. As the “deepest man on Earth” it took Nitcsh 4 minutes and 20 seconds to dive down to an incredible 214 metres and return to the surface.
Nitsch’s total world record breath hold time of 4 minutes and 20 seconds was a lot shorter than Ricardo da Gama Bahia static diving record of 20 minutes and 21 seconds. So why Nitsch couldn’t he dive any deeper? Just like in static breath holds, free divers train themselves to force extra air into their lungs (lung packing) and the diving reflex redirects their blood (which contains the oxygen they need to survive) is away from non-vital areas (like fingers and toes) to important areas like the brain . The big difference between free and static diving is water pressure which increases by 1 atmosphere ever 10 metres (33 feet).
The human body is largely made up of liquids (mostly water, ~70%) and solids which don’t compress (or squish) very easily with increased pressure. The lungs contain air which is a gas and does compress. In fact an old dude called Boyle had a law that explains that the volume of a gas is inversely proportional to the pressure exerted on it. While this may sound a little like gobbledygook, it means that at 10 metres deep where the water pressure is 2 atmospheres the free divers lung volume will be ½ (3 litres) of what it was at sea level (6 litres). Crazily, at 200 metres Nitcsh’s lung volume would have been ~1/21 the size of what it was a sea level! You’ve probably felt this squishing of air before if your ears have ever hurt when you’ve dived down to the bottom of the pool. Imagine it 200 metres down!
to breathe, or not to breathe 
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