A family from Portsmouth captured on video a shark eating a dead grey seal off the south tip of Monomoy Island in Chatham.
A state shark expert confirmed for the Cape Cod Times that the shark was a great white.
Peter Mottur posted the video on YouTube.
The family spends time boating off Monomoy Island, which is known as a hang-out for seals and their massive predators.
The shark was apparently unfazed by nearby boats.
The shark expert told the newspaper that the video is rare and could be very useful to scientists, not just an unforgettable moment for the family.