A man plunged over the Canadian side of Niagara Falls on Monday, dropping more than 55 metres into the crashing waters below, but survived.

Only three or four people are known to have lived after being swept over the falls without a protective device.

The man, believed to be in his 30s or 40s, suffered life-threatening injuries to his chest and head, and is now being treated in a Hamilton hospital.

Witnesses told Niagara Parks Police the man climbed over a retaining wall about 10 metres above Horseshoe Falls around 10:20 a.m. and deliberately jumped into the Niagara River.

He was carried by the strong current over the falls and witnesses reported seeing him surface at the bottom, adjacent to the Journey Behind the Falls observation platform, police said in statement released Monday.

A parks police officer found the man along the rocky shoreline just as the victim collapsed in waist-high water, police said.

The Niagara Falls fire department used an aerial truck to lift the man from the gorge to a waiting ORNGE air ambulance around 12:16 p.m.

Fire department Platoon Chief Dan Orescanin said it was difficult for rescue workers to reach the man.

"It's a very difficult slope. It's a very difficult place to traverse," he said.

The man, believed to be in his 30s or 40s, was taken to a Hamilton hospital with life-threatening injuries.

The last person to go over Horseshoe Falls and survive was a 30-year-old Canadian man in March 2009.

A Michigan man survived the plunge in 2003.