* This story is featured in BBC Future’s “Best of 2019” collection. Discover more of our picks.

There was a sickening crack when the thick cable connecting Chris Lemons to the ship above him snapped. This vital umbilical cord to the world above carried power, communications, heat and air to his diving suit 100m (328ft) below the surface of the sea.

While his colleagues remember the terrible noise of this lifeline breaking, Lemons himself heard nothing. One moment he was jammed against the metal underwater structure they had been working on and then he was tumbling backwards towards the ocean floor. His link to the ship above was gone, along with any hope of finding his way back to it.

Most crucially, his air supply had also vanished, leaving him with just six or seven minutes of emergency air supply. Over the next 30 minutes at the bottom of the North Sea, Lemons would experience something that few people have lived to talk about: he ran out of air.

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“I’m not sure I had a full handle on what was happening,” recalls Lemons. “I hit the sea bed on my back and was surrounded by an all-encompassing darkness. I knew I had a very small amount of gas on my back and my chances of getting out of it were almost non-existent. A kind of resignation came over me. I remember being taken over by grief in some ways.”