Well, it sounds scary

 

The short phrase "mutant mosquito" sounds, potentially, like a really scary thing. 

Apparently it's not that scary, but there were concerns that the mosquitoes in the London subway, aka The Tube, were mutants.

And apparently they are, but they mutated a lot earlier than has been thought for quite awhile. They are now found in many underground environments, subways and the like, around the world, with the sinister name Culex pipiens f. molestus. 

So where did it come from?

"Now, an international team of over 100 scientists has traced the mosquito’s origin to a time far before the advent of artificial underground systems, raising further questions about its evolution.

The researchers assessed nearly 350 contemporary and historical mosquito DNA samples and found that C molestus evolved first to live and breed above ground in the Middle East over 1,000 years ago."

(from Study traces origin of mutant London underground mosquito to Middle East)

So it didn't mutate in the Tube. But that's still a good story. It definitely still lives there, and in a lot of other subways, too.