Grey whales grow to almost five metres long and can live for 60 years. There used to be three major populations of grey whale - in the western and eastern North Pacific Ocean, and in the North Atlantic.
Now, there are only about 160 of the mammals living off the coast of South Korea, and about 20,000 to 22,000 whales that migrate between California and Arctic waters.
But they were hunted to extinction in the North Atlantic about 300 years ago, and there have been no sightings of them in the Atlantic since.
So when one of the whales appeared off the coast of Israel last month, scientists were puzzled as to how it got there. A few weeks later, it was spotted off the coast of Spain, about 3,000 kilometres from its last sighting.
Scientists compared photos from both sightings and saw the same marks on the tail, showing that it was the same whale.
I saw grey whales last summer when I sailed from the North Pacific through the Northwest Passage. They are exciting to watch when they surface to blow spray high in the air.
Scientists say that either the whales in the Atlantic Ocean had survived and started breeding again, or that a single grey whale made the longest ever migration, swimming from the North Pacific, over the top of North America through Arctic waters and down into the North Atlantic.
Scientists think during the summer feeding season north of Alaska, the whale wandered off and began swimming through the Northwest Passage.
This passage is covered by thick ice all winter, and the ice only breaks up in the summer. Warmer temperatures related to climate change mean the passage has less ice every year, allowing the whale to find an open route to the Atlantic.
This would mean the whale travelled about three times as far as it would during its normal migration. If so, other whale migration patterns may be affected by climate change.
Cameron is available to speak to primary and secondary students about environmental and climate change issues as well as his recent Arctic sailing expedition. Contact firstname.lastname@example.org