Dean Potter knew the risks every time he flew off a cliff in a wingsuit.

The extreme athlete lost a friend to a BASE jumping accident last year, and spoke about the death-defying nature of the sport at that friend's memorial service.

Potter was one of two men killed after jumping from a 2,300-meter promontory called Taft Point in Yosemite National Park. Some media reported that the pair smashed into a rocky outcrop when they tried to fly through a narrow gap in the mountains.

Potter was renowned for his daring and sometimes rogue climbs as well as BASE jumps. He made headlines worldwide in April 2012 when he walked a tightrope without safety equipment across the 200-meter-deep Enshi Canyon in Hubei province, covering the 41 meters in about three minutes.

On Saturday, someone called for help after losing contact with Potter, 43, and his climbing partner, Graham Hunt, 29.

The men wore wingsuits - skintight suits with batwing sleeves and a flap between their legs - to help them glide. However, parachutes designed to slow their descent had not been deployed, park ranger Scott Gediman said.

BASE stands for buildings, antennas, spans (such as bridges), and Earth (such as cliffs and mountaintops) that jumpers can parachute from. The sport is illegal in all US national parks, and it was possible the men jumped at dusk or at night to avoid being caught by park rangers.

"BASE jumping is the most dangerous thing you can do. ... Every time you jump, it's a roll of the dice," said Corey Rich, a photographer who documented some of Potter's feats. "The odds are not in your favor, and sadly, Dean pulled the unlucky card."

Potter is famous for pushing the boundaries of climbing by going up some of the world's most daunting big walls and cliffs alone, using his bare hands and without ropes. He took the sport to an extreme level with highlining - walking across a rope suspended between towering rock formations while wearing a parachute for safety in the event of a fall.

He drew criticism in May 2006 after he made a "free solo" climb of Utah's iconic Delicate Arch in Arches National Park.

AP - AFP