In any given year, over 1.3 billion passengers zip beneath London on its fabled Underground—the world’s first subway system. But something else lurks in the Tube’s quick-moving depths: a subspecies of mosquitoes that, the BBC’s Katie Silver reports, evolved inside the London Underground.

The appropriately-named Culex pipiens molestus came to be over the Underground’s 150-year history. Silver writes that it was first reported during World War II, when people who used Tube stations as bomb shelters learned that the depths held plenty of pests. Among the nuisances were mosquitoes with a nasty, irritating bite.

In 1999, an English researcher named Katharyne Byrne went underground to investigate further. When she compared Underground mosquitoes and compared them to others found in London houses, she learned that they were a distinct subspecies.

After ruling out migration from elsewhere in the continent, Byrne concluded that the London Underground was colonized by mosquitoes a single time, then achieved “reproductive isolation,” or barriers to reproduction with different species, in the subway tunnels.

The quick separation of mosquitoes into their own, subway-dwelling subspecies is an example of quick-moving speciation (the process by which animals evolve into distinct species). Darwin’s finches in the Galapagos are often cited as an example of lightning-fast speciation—since they’re so remote, they remain genetically isolated and adapt rapidly.

Silver reports that some scientists doubt the mosquitoes are really unique to the underground. In 2011, for example, a mysterious invasion of the mosquitoes was found in New York sewers.

More up-to-date research would need to be conducted to figure it out for sure. Consider this a call to would-be researchers whose interests include both long train rides and calamine lotion: Your future in Tube-related evolutionary research could be bright indeed.