New York City police are investigating another unusual incident involving the city's underground sewer system after three men were reportedly seen climbing into a manhole in Manhattan early Sunday morning.

The latest incident occurred shortly before 1 a.m. on West 141st Street in Hamilton Heights, near Amsterdam Avenue. According to police, a witness reported seeing the three individuals enter the underground system, but responding NYPD officers were unable to locate them.

The incident is the first of its kind reported in Hamilton Heights, although similar mysterious sightings have occurred in several New York City neighborhoods in recent months.

Just two days earlier, five individuals, including four men and a woman, were spotted emerging from a manhole on East 82nd Street on Manhattan's Upper East Side. The group reportedly left the area in a black SUV before police arrived, with investigators later discovering shovels and other equipment nearby.

In another incident last month, two individuals were arrested after allegedly entering a manhole on East 61st Street during the United Nations General Assembly, when security measures across Manhattan were heightened.

Officials with the city's Department of Environmental Protection believe some individuals may be entering the underground sewer network to search for valuables and scrap metal, including items washed into drainage systems during storms.

Authorities have not established whether the recent incidents are connected or whether the individuals involved were searching for valuables.

The NYPD continues to investigate the incidents as questions remain about the repeated unauthorized entries into the city's underground infrastructure.