Somewhere beneath the mounds of snow covering Washington, D.C.’s Capitol Hill district, just steps from the Library of Congress, a cluster of tropical mosquitoes capable of carrying Zika and other viruses is holed up, waiting for spring.

Washington, D.C. is well beyond the comfort zone for these mosquitoes—this species can’t survive winters north of Alabama and South Carolina. But this colony has somehow hung on for four years, genetic evidence shows, making it the northernmost permanent outpost for the disease-transmitters Aedes aegypti known to date.

This particular colony is small, virus-free, and poses no health threat, say the scientists who’ve been monitoring it. But its existence shows the species is adapting to cold climates and conquering new territory: another sign of the growing threat that insect-borne diseases pose in a rapidly changing world.

Looking forward, climate change is a clear risk for burden of a variety of infectious diseases.

“It’s still amazing to me,” says entomologist David Severson, a Notre Dame University professor and member of the research team that last week released news of the underground mosquitoes. “Clearly the climate of Washington D.C. is not hospitable enough for them to survive above ground.”

These mosquitoes may be sheltering somewhere like a metro tunnel or storm sewer, Severson says. This enterprising behavior could put them one step closer to dense northern populations. Aedes aegypti mosquitoes have been found this far north in summers before, but were likely imported by travelers, and none have become established year-round. Severson thinks it’s likely that other colonies are in the area but just haven’t been found yet.