Commercial diver Michael Packard, 56, said he was about 14 metres deep in the waters off Provincetown, Cape Cod, when "all of a sudden I felt this huge bump, and everything went dark".

He told broadcaster WBZ-TV he thought he had been attacked by a shark, common in the area's waters, but then realised he could not feel any teeth and he wasn't in any pain.

"Then I realised, oh my God, I'm in a whale's mouth ... and he's trying to swallow me," Packard said after he was released from hospital on Friday.

"And I thought to myself OK, this is it - I'm finally - I'm gonna die." His thoughts went to his wife and children.

He estimates he was in the whale's mouth for about 30 seconds, but continued to breathe because he still had his breathing apparatus in.

Then the whale surfaced, shook its head, and spat him out. He was rescued by his crewmate in their boat.

It was initially reported that Packard broke his leg in the incident, but he later clarified that his legs were only bruised.

Whale expert Charles "Stormy" Mayo of the Center for Coastal Studies in Provincetown told the Cape Cod Times such human-whale encounters are rare.

Humpbacks are not aggressive and Mayo thinks it was an accidental encounter while the whale was feeding on fish, likely sand eel.

Australian Associated Press