New York police are investigating something that sounds like the opening of a film: groups of people going in and out of the city's sewer manholes at night. For now there's no threat to public safety - but the mystery is strange enough to warrant an investigation.

The first case was reported last Thursday around 11 p.m. in Gravesend, Brooklyn - eight people lifted a manhole cover and the same group came out about three hours later. Just two hours after that, a similar scene in Williamsburg: a few people went in and emerged after more than two and a half hours. Another case in May, in Astoria, Queens, involved people with headlamps and fishing waders.

The police checked and found nothing malicious. "Our highly trained Emergency Service Unit members examined the sewer system. They checked whether anyone had left something dangerous, but found nothing," the police said. Their theory is down to earth: the people are probably looking for valuables that ended up in the drains.

There's something almost Balkan about this story - that old belief that somewhere, even underground, something valuable is waiting for whoever dares to go down. Whether it's gold, lost rings or just an urban legend that made a few people buy fishing waders, the police still don't know. But one thing is certain: even in the world's biggest city, the hunt for hidden riches never goes out of fashion.