Video posted to social media shows people in overalls and what appears to be headlamps emerge from the manhole, one by one.
It happened on Tuesday around 4 a.m. The NYPD says it is aware of the video.
The Department of Environmental Protection inspected the sewer system and did not find any structural damage.
The video follows similar underground sightings in Brooklyn and Queens earlier this year. It is unclear whether all three incidents are connected.