Looking down, he could see a dorsal fin in front of his feet as he stood on what he described as 10 feet of back as wide as his surfboard and as black as his own Neoprene wetsuit.

A tail thrashed back and forth and the water churned around him like a depth charge went off. "It was pretty terrifying just seeing the shape emerge out of nothing and just being under me," he said. "And the fin coming out of the water. It was just like the movies."

The several seconds Niblack spent on the back of the great white Monday off Seaside, Oregon, was a rare encounter, though not unprecedented, according to Ralph Collier, president of the Shark Research Committee in Canoga Park, California, and director of the Global Shark Attack File in Princeton, New Jersey.

He said he spoke to a woman who was kayaking off Catalina Island, California, in 2008 when a shark slammed her kayak from underneath and sent her flying into the air. She then landed on the back of the shark, Collier said.

"At that point the shark started to swim out to sea, so she jumped off its back," Collier said.