Video footage, taken on Monday evening, shows what appears to be the fin of a shark moving through the River Thames near Hammersmith Bridge in London.
The animal hasn't been spotted since. The big question: is it really a shark?
Sharks have been spotted in the River Thames before.
"We do have several species of shark in UK waters, and you can find them around the estuary area," says Jack William, Senior Aquarist at Sea Life London Aquarium.
The Zoological Society of London add that "sharks are a key part of the natural biodiversity [of the Thames]."
But "it's highly unlikely" to be a shark, says William. "That far up the Thames the water is extremely fresh. It doesn't start getting salty until about Chelsea."
Most shark species need salt water to stay alive, so it's "unlikely that any sharks would swim that far upstream," says The Port of London Authority, the trust that governs the Port of London.
If it is a shark, it's thought it could be a tope shark, reported Helen Drew during a BBC investigation. Harmless to humans, this slender and elegant shark species is often found close to shore all around the UK and can grow up to 6 feet long. The species "has been seen in the Thames Estuary before," says Drew.
It's not the first time large marine animals have been spotted in the Thames before. The Port of London Authority says the Thames has an "abundance of aquatic life with occasional visits from dolphins, porpoises and even whales."
Main image: fin of a shark – not the individual seen in the River Thames. Credit: Getty
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