A series of bizarre sightings of people popping in and out of New York’s vast, subterranean sewer system in recent days has the whole city wondering what exactly is going on.

There’s no threat to the public and officers have scoured the sewer system to make sure “nothing nefarious had been left behind,” a law enforcement source said.

Since May 5, at least three of the unexplained sewer incidents have been recorded in Brooklyn and Queens.