Sydney  - Grey nurse sharks, giant cuttlefish, glistening anemones and bull rays make Fish Rock Cave on Australia's east coast one of the world's best scuba dives.

Daredevil aquanaut Mike Wells said Monday he hoped to set a world record for freediving by swimming the length of it in a single breath.

That entails swimming to a depth of 14 metres to get into the 120- metre tunnel, then negotiating a tricky exit where the gap in places is no wider than Wells' 1-metre fins, before popping up the 24 metres back to the surface.

The swim should take around three minutes, mostly in pitch black and at breakneck speed.

"When you're moving through quite quick the biggest challenge is not getting lost and finding yourself in a dead end," he said.

Dead end: quite literally, really. Although there will be five scuba divers along the route helping light the way and ready to share their air supplies if the mission has to be aborted again, oxygen debt can come quickly when you're holding your breath. Blacking out could be deadly.

Wells tried at the weekend, getting three-quarters of the distance before strong currents forced him to abandon the attempt.

"The surge was so powerful that I was like a salmon swimming upstream," he said.

For Wells, 29, a professional diver who began swimming distances underwater on lung-power alone 19 years ago, Fish Rock Cave is a second home - and its populace of placid grey nurse sharks a second family.

"I actually get to recognize some of the sharks," he said. "I recognize their markings and know them individually. Hopefully, they'll recognize me and let me get on with it." (dpa)