In a water temperature fractionally above zero, Williams wore only a wetsuit and carried a torch.
“It was a super-challenging dive,” he said afterwards. “The brutal cold and the darkness were far beyond anything I have experienced in the sport. It was a relief to be back at the surface.
“I felt relaxed on the way down, but the swim up was tough going. I think I was over-weighted, and when my dive response kicked in at depth, my legs felt like lead.
“It took me nearly 30 seconds longer than normal to swim up. This is common with no-warm-up diving, when you don’t get a dive response before your deepest dive.”
Williams lives in Melbourne, Australia and trained for his record bid off the Victorian coast, in New Zealand’s Lake Taupo and in Finland.
The previous record of 65m was achieved in 2015 by Russian Konstantin Novikov.
Williams was set to try to improve on his own 70m dive today (29 March) with a descent to 75m.