Marine biologists are scratching their heads this week, trying to figure out how a grey whale, normally found only in the Pacific Ocean, has been spotted off the coast of Israel.

The solitary whale, measuring some 12 metres (39 feet) and weighing around 20 tonnes, was first sighted off Herzliya on the coast of the Mediterranean, on Saturday.

Lance Barrett-Lennard, of the Department of Zoology at the University of British Columbia, knows his whales and says he has never heard of a grey in the Mediterranean. He says when he first heard the report, he was skeptical.

"Well, I initially was thinking and I'm still -- the jury is still out in my mind -- that it was some sort of hoax that perhaps somebody doctored some photos," he said.

But Barrett-Lennard says the group that revealed the discovery, the Israel Marine Mammal Research and Assistance Center, is reputable.

The chair of the centre, Dr. Aviad Scheinin, says the discovery is stunning.

"It's an unbelievable event which has been described as one of the most important whale sightings ever," said Scheinin told reporters earlier this week.

Scheinin says grey whales once inhabited the North Atlantic, but became extirpated (locally extinct) more than 200 years ago. He says as far as he knows, there haven't been any grey whales in the Atlantic since the 18th century.

How the whale found its way from the Pacific to the Atlantic and then into the Mediterranean is a mystery. It presumably travelled thousands of kilometres after losing its way in search of food. The best guess from Scheinin is global warming played a role.

Every spring, grey whales head north to the icy Arctic Ocean to feed. But the ice there has been melting at an alarming rate. So the theory is that after the whale left its mating grounds near Mexico, it travelled to the North Pacific, then travelled though the now open Northwest Passage and found itself in the Atlantic.

From there, it must have kept going, turning left when it reached Gibraltar, into the Mediterranean, before finding itself off the shores of Israel.

"If it did in fact go through the Arctic, it would be one of a number of species that are recolonizing the Arctic where they've never been known before in human memory," says Barrett-Lennard.

Still, Barrett-Lennard says it's unlikely we'll ever see another grey in the Mediterranean Sea, since migratory mating patterns are ingrained. In fact, he wouldn't be surprised if this whale attempts to make its way back to the Pacific.

Because while the whale could survive in the Mediterranean, right now, it's the loneliest whale in the world.