Mole people? Mario brothers? Ninja turtles? Bizarre sightings of people entering ad leaving the city’s vast sewer system has left residents wondering what’s going on and sparked a police probe.

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.