A humpback whale thought to be up to 10 metres (33ft) in length has been spotted in the River Thames.
British Divers Marine Life Rescue (BDMLR) said it was "definitely a humpback".
Although initially it didn't show any signs of distress, it has since died, possibly because it couldn't find enough food.
The last whale spotted in the Thames was a different type of whale called a beluga whale.
When whales or dolphins go off course they can end up in rivers like the Thames, or worse they can end up getting stuck on dry land or "beached".
Marine scientists don't know exactly why this happens but there are some theories.
One theory is that they get sick and are pushed in shore by currents. Whales use sonar to find distant objects and sense their size and shape.
Another reason why marine mammals like dolphins and whales might get lost is because underwater technologies used by ships send out competing sonar signals which can confuse them.
Changes in the environment can also cause whales to behave differently. If sea temperatures rise, food stocks get low or the water is polluted they might change course.
Sometimes whales or dolphins can make a simple mistake and head in the wrong direction.
Because whales and dolphins travel in groups and are very sociable animals, when one makes a mistake and heads the wrong way, others might follow. This is why sometimes a number of whales can get beached at the same time.