Experts from the Natural History Museum were called to Tower Bridge this week when a harbour porpoise became stranded on the Thames foreshore.
Unfortunately the animal had already died.
There are around 300 cases of porpoise strandings along UK coasts every year and they appear quite regularly in the Thames.
Dolphin pods swim in occasionally but tend to manage to turn around and take themselves out again. A dead humpback whale came down the Thames in March this year.
In 2006 an 18-foot whale became stranded in the Thames and died after a massive rescue attempt. Its skeleton is now used for research at the Natural History Museum.
If the porpoise recently found in the Thames is in good enough condition, it may be used to provide data on how porpoise populations are changing.