Have you ever been on the subway and seen something that you did not quite recognize, something mysteriously unidentifiable?

Well, there is a good chance scientists do not know what it is either.

Researchers at Weill Cornell Medical College released a study on Thursday that mapped DNA found in New York’s subway system — a crowded, largely subterranean behemoth that carries 5.5 million riders on an average weekday, and is filled with hundreds of species of bacteria (mostly harmless), the occasional spot of bubonic plague, and a universe of enigmas. Almost half of the DNA found on the system’s surfaces did not match any known organism and just 0.2 percent matched the human genome.

“People don’t look at a subway pole and think, ‘It’s teeming with life,’ ” said Dr. Christopher E. Mason, a geneticist at Weill Cornell Medical College and the lead author of the study. “After this study, they may. But I want them to think of it the same way you’d look at a rain forest, and be almost in awe and wonder, effectively, that there are all these species present — and that you’ve been healthy all along.”

Dr. Mason said the inspiration for the study struck about four years ago when he was dropping off his daughter at day care. He watched her explore her new surroundings by happily popping objects into her mouth. As is the custom among tiny children, friendships were made on the floor, by passing back and forth toys that made their way from one mouth to the next.