Researchers flying south of Nantucket on Friday were stunned to spot a gray whale, a species that had vanished from the Atlantic Ocean by the 18th century, officials said.
A team of scientists from New England Aquarium’s aerial survey team were about 30 miles south of the island when they sighted the animal, which dove into the water and resurfaced repeatedly as if it were feeding, the aquarium said Tuesday.
“On these surveys, we generally expect to see . . . humpbacks, fin whales, North Atlantic right whales, and so on,” Orla O’Brien, associate research scientist at the Anderson Cabot Center for Ocean Life at the aquarium, said in an interview.
“But, very unexpectedly, we happened to see a gray whale,” said O’Brien, who has been conducting aerial surveys since 2011 but had never encountered one of the sea mammals before.
The whale, estimated to be roughly 40 feet long, was first spotted by Kate Laemmle, a research technician in the aerial crew, which then flew closer to the creature. At first, the whale was hard to see clearly as it dove into the water and briefly surfaced, though it was obviously not one of the animals commonly sighted in the area.
“We knew immediately that there was something different,” O’Brien said. “It just didn’t look right. We had originally thought it was a right whale, maybe, because it didn’t have a dorsal fin, but it looked very pale and mottled.”
The plane circled the area for about 45 minutes as researchers took more photos, which confirmed that the creature was a gray whale.
“It was just kind of this crazy feeling, because looking at the photos, it was undeniable that it was a gray whale, but we know that gray whales don’t exist in the Atlantic,” O’Brien said. “So it was just this cognitive dissonance, almost. It’s like, I know what I’m looking at, but I can’t believe my eyes.”
The crew continued taking photographs so they could prove their finding. “It’s the kind of sighting that, if you don’t have definitive photos, no one’s going to believe you,” O’Brien said.
Laemmle was also shocked, she said in a statement from the aquarium.
“My brain was trying to process what I was seeing, because this animal was something that should not really exist in these waters,” Laemmle said. “We were laughing because of how wild and exciting this was — to see an animal that disappeared from the Atlantic hundreds of years ago!”
The crew stayed in the area for a while to observe the animal’s behavior.
“It was being pretty consistent, spending five or six minutes underwater, coming up for three breaths, and then diving back down,” O’Brien said. “I’m not a gray whale expert, but just based on my history with whales and whale behavior and observing them, it looked to me like it was feeding, which is just really interesting. It shows that whales are very adaptable.”
Gray whales are common in the North Pacific, with a large population living off the West Coast and migrating up and down the shoreline, and are easily distinguished from other whales because they have a dorsal hump and no dorsal fin, and their skin is mottled gray and white, the aquarium said.
The beasts had disappeared from the Atlantic Ocean by the 1700s, but there have been five sightings in the Atlantic and Mediterranean in the past 15 years, including one off Florida in December that is believed to be the same whale spotted near Nantucket last week, officials said.
Scientists believe climate change is behind their return. In recent years, the sea lane connecting the Atlantic and Pacific oceans through the Arctic Ocean has routinely been partially ice-free during the summers, in part because of rising temperatures around the globe.
That has allowed some gray whales to become world travelers and explore a new ocean, researchers said.
The sighting of a gray whale close to Massachusetts is an exciting discovery, O’Brien said, but also a reminder that local waters are home to many other kinds of whales and sea creatures. Boaters and others should take caution, she said.
“It’s important for people to just remember that, and if you’re out on the water, just be careful and just be mindful that we have a lot of marine animals besides gray whales around here,” she said.
Jeremy C. Fox can be reached at jeremy.fox@globe.com. Follow him @jeremycfox.