KALAMATA, Greece May 26, 2012— -- William Winram makes it sound so easy. "You're going to hold your breath, pull yourself down a line under the sea," says the champion deep diver from Canada.

Tribe of ‘fish people’ boast extra-big spleens for deep sea diving By . Jon Lockett, The Sun. Published April 20, 2018. Updated April 20, 2018, 7:10 p.m. ET. AFP/Getty Images ...

So the most tricky part of a deep dive is the last stage of the ascent, when there is the risk of a shallow water black-out as the pressure fades and the oxygen levels in our tissues suddenly drop.