buoyancy

“When you change your body and you become stronger, it changes everything inside the water, your buoyancy, your resistance,” he says.

The brothers had several things in their favour, Geoff says, including being young and fit and, crucially, they were wearing wetsuits, which provided invaluable buoyancy aid.

Back in the pool, divers are constantly adjusting Rosemary's buoyancy in the water to make the experience as close as possible to microgravity.

Yet incredibly, sealing the ends watertight and fitting them with ballast tanks, gives enough buoyancy to tow them behind tugboats.

In essence, it was a big raft, with tall, reinforced sides on 60 empty steel drums for buoyancy, towed by motorboat.