Mysterious fin spotted at Drakes Beach likely belongs to a whale

SAN FRANCISCO -- Was it a shark on the prowl? A killer whale stalking prey? Or something far less nefarious in the water?

The National Park Service snapped a picture at Drakes Beach in Marin County and then took to Twitter to see if anyone had the answer.

At first glance it looks like a giant fin sticking out of the water. This of course generated all kinds of responses and speculation as to what creature may be lurking out there.

But the park service told ABC7 news it has a pretty good idea who the mystery fin belongs to. The photo was taken at the same time four gray whales were seen spouting in the area.

They say the fin is likely half of a gray whale's fluke.

RELATED: Gray whale likely on migratory path to Alaska spotted in San Francisco Bay

This is a good time of year to see the whales since they are migrating north along the California coast to Alaska. Scientists say the whales like to conserve their energy and tend to stay along the coast rather than out at sea.

Gray whales are the marathon racers of the ocean. It is believed their migration, which can exceed 13,000 miles, is the longest of any mammal on earth.

See more stories and videos about whales.