This is the terrifying moment a thirty-foot basking shark hit a fishing boat off the coast of Portland, Maine in the USA.

In the clip, which was filmed on June 10, the enormous shark swims directly at the boat and then hits it with a loud crash.

"We were jigging for mackerel when we saw a large dorsal fin coming towards our boat," the filmer Dustin Beamon wrote online.

"We were just thinking it was going to be your typical shark until it got closer and we saw the size of it and our first thought was that it was a great white," Beamon said.

"Our boat is 22 feet long and this basking shark was at least 10 feet longer than our boat, it could have easily capsized us," he added.

"As it swam under our boat it bumped it, knocking almost everyone in the boat off their feet."

Basking sharks are the second largest fish in the world after the whale shark. Despite their huge size, they are placid filter feeders and do not attack humans.