A mysterious object - or creature - has been spotted 'swimming' in the River Thames near the 02.
Footage shows the object break through the water's surface before going back down again in Greenwich, reports ITV News.
See also: Mystery 'sea monster' washes up on beach in Mexico
See also: Strange creature washes up in aftermath of Japan tsunami
The video was taken by a passenger on the Emirates Air Line cable car, and the caption read: "This was on the cable car in Greenwich yesterday. something huge was moving under the water and then briefly surfaced. Are there whales in the Thames?? Or is it some weird submarine?"
The Mirror reports that some people believe it could be a lost sperm whale.
Back in 2006, the paper adds, a young female Northern bottlenose whale was discovered swimming in the river.
According to the Evening Standard, 49 whales, over 2000 seals and 450 porpoises and dolphins have been seen in the River Thames in the last 10 years.
Canary Wharf is the most likely place to see them because of its high vantage points.
Joanna Barker, European conservation projects manager at Zoological Society of London said (ZSL), said: "People are often surprised to hear that marine mammals are regularly spotted in central London.
"As top predators, their presence is a good sign that the Thames is getting cleaner and supporting many fish species.
What do you think the latest sighting is?