On Thursday morning, Kiwi freediver William Trubridge will embark on his second attempt to beat his own world record for unassisted freediving.
At Deans Blue Hole in the Bahamas - the deepest blue hole on earth - he will attempt to dive to 102m in a single breath, with only a single pull on a rope allowed to stop his descent and enable him to come back to the surface.
Competitive freediving is a relatively new sport with very old roots.
Ancient cultures used the technique to harvest food or resources. In Japan, deepwater divers have been gathering pearls for 2000 years.
Basically, freediving is doing something underwater in one single breath:
But the most jaw-dropping is endurance freediving.
Until 1960, scientists believed going deeper than 50 metres would crush your ribcage. Luckily, they got that wrong.
Professional freedivers - like Trubridge - have a discipline called "free immersion", using just a rope to go up and down. No fins, no sled.
Trubridge also holds the record for that: 121 metres deep on a single breath.
That's 36 stories, straight down.
And, by the way, you're also slowly suffocating.
The "rule of threes" for survival says death will knock on your door after:
Freedivers routinely bust that last one.
Once a diver holds their breath and starts descending underwater, it's not the lack of oxygen that's the problem but rather the buildup of excess carbon dioxide.
Provided they learn to tolerate the discomfort associated with this, they'll be able to hold on for around 60 to 80 percent longer than usual.
A 102m Trubridge freedive will take over four minutes on one breath.
The freediving record is around 12 minutes!
The way back up is also life-threatening. Among other things, oxygen begins streaming out of a diver's blood, which can lead to blackout.
But once safe, freedivers often break the surface with a smile - and with plans to go back down again, as soon as possible.