A mystery remains in Gravesend after a group of people was caught on video coming out of a sewer system last week.

It happened early Friday morning on McDonald Avenue near Colin Place. The video from Flatbush Scoop shows seven adults exiting the manhole.

The Department of Environmental Protection says its inspection found no damage to the system. Days later, neighbors are still wondering why anyone would go down there.

"I thought it was, you know, maybe kids fooling around, but then I saw the video, and it seemed like it wasn't kids. They were adults. So it looks a little suspicious, in my opinion," said one passerby.

DEP is also reminding people that not only is entering a manhole dangerous because of potential deadly gases and hazards, it's also illegal.

No arrests have been made.