An American man slid into the Niagara river and plunged over the falls, becoming the first person to survive the journey without any kind of protective equipment.
Police said yesterday they would charge Kirk Jones, 40, from Michigan, with illegally performing a stunt.
Tourists on the Canadian side of the river were startled on Monday when the fully clothed Mr Jones was spotted floating on his back towards the lip of the falls. He was said to be smiling. Witnesses said he went over the edge and, after disappearing for several seconds, was spotted swimming towards rocks at the bottom.
"He just looked calm. He just was gliding by so fast. I was in shock really that I saw a person go by," said Brenda McMullen, who was visiting the falls from Ohio. "I saw him disappear over the edge," her husband, Terry, added.
The only other person known to have survived the 180ft waterfall was a seven-year-old boy who was thrown from a boat above the falls in an accident in 1960. He was wearing a lifejacket. About 15 thrill-seekers and daredevils have attempted the drop since 1901, in barrels and other contraptions. Only 10 survived. Depending on the season, about 150,000 gallons of water crash over the falls every second.
Last night Mr Jones was being treated for minor injuries and undergoing psychological tests. Police said they were ruling out attempted suicide and treating it as an intentional act.
Brian Merrett, chairman of Niagara Parks Commission, called the stunt "stupid".
"Our people went down in the gorge and got him. That is why we don't condone this. It puts all of our people at risk to do the rescues. That is why we are adamant about stunting."
Running the falls for fun is strictly forbidden and anyone attempting it can be fined $10,000 (£6,000) - assuming they survive the ordeal.
Lynda Satelmajer, from Ontario, described seeing the man get into the water at the top of the falls. "He seemed a bit edgy," she said. "He walked over to where we were standing and he jumped and slid down on his backside and went over the brink.
"It was really freaky, actually. He was smiling."