Two blue whales were seen off the coast of New England, in a rare sighting of the earth’s largest mammal.

Blue Ocean Society for Marine Conservation co-founder Dianna Schulte saw the whales off Rye Harbor, New Hampshire on Friday, while she was working aboard the Granite State, the Associated Press reported.

The group’s executive director, Jen Kennedy, said seeing two blue whales—which can be up to 100 ft. long—swimming together is very rare.

“To spot two blue whales together is simply unheard of,” she told WMUR-TV. “In the Atlantic, they are usually sighted off of St. Lawrence in Canada.”

Blue whale sightings occur maybe every five to ten years, Kennedy said. “We’ve never seen two together in our last 20 years of whale watching and research off the N.H. coast.”

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Write to Mahita Gajanan at mahita.gajanan@time.com.