Has a dolphin been spotted swimming in the Thames, or is it something else?
Footage filmed in Richmond, southwest London appears to show a dorsal fin poking above the water before diving below the surface.
British Olympic swimmer Andy Jameson, who won bronze at the 1988 Seoul Games, revealed the sighting on Twitter.
"Ok .... you may all think I'm mad, but I just saw a Dolphin in the river Thames at Richmond," he tweeted.
But social media users were quick to point out the creature was more likely to be a porpoise, which have smaller noses and bodies and are more common in the capital.
An August 2015 survey by the Zoological Society of London found marine life was thriving in the Thames, with regular sightings of whales, dolphins, porpoises and seals in the river.