River monster in the Thames, London, England

Posted by an eyewitness: 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

Location of sighting: Greenwich, London, UK
Date of sighting: 26 March 2016

In the Thames, an eyewitness noticed a strange creature hidden under the water.
Based on direction of motion of creature, it was swimming upstream toward Buckingham Palace.
Silhouette which looked through from under the water, similar to the plesiosaur.

detailed video

The Plesiosauria (/ˌpliːsiəˈsɔːriə, -zi-/;Greek: πλησίος, plesios, meaning “near to” and Sauria) or plesiosaurs are an order or clade of Mesozoic marine reptiles (marine Sauropsida), belonging to the Sauropterygia.
https://en.wikipedia.org/wiki/Plesiosauria
Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 205 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution.

3D animation showing the most likely swimming motions

Restored skeleton of Plesiosaurus

Cast of “Plesiosaurus” macrocephalus, yet to receive a valid genus name