Created by Dutch artist Florentijn Hofman, HippopoThames it is a 21 metre long hippopotamus currently wallowing in the river just above Vauxhall Bridge.
Hofman is famous for creating the giant yellow inflatable ducks that have been spotted around the world in harbours from Baku to Hong Kong, sometimes deflating.
The hippo is a bit more substantial wooden construction so I think its safe from the odd pin-prick.
It is part of the Thames Festival this month and celebrates the discovery of hippo remains in central London.
Yes, that's right, 100,000 years ago hippos lurked in the Thames and fossilised bones were found in Trafalgar Square as described here.
The spot where it is moored was probably selected as being the most central place out of the way of busy city centre river traffic, but it has an interesting ancient historical connection.
Vauxhall is where one of the oldest archaeological structures has been found within London. On the river bank at very low tide it is possible to make out the remains of a causeway (or even bridge) constructed around 3,500 years ago. Recently remains of an even older structure - possibly a staggering 6,500 years old - have been found downstream in front of the MI6 building.
However to be honest what it makes me think of is something else that the Thames is famous for, namely mud, and in particular the classic song by Flanders and Swann. Feel free to join in....
Updated: a list of other things strange things to have been floated down the Thames here.