Listen to the strange hum of a 40-year-old art installation hidden inside a Times Square grate.

In the world’s most public space, one feature is known for being unknown. It's a hum-—a strange, hard-to-identify sound resembling a distant moving and clanking of heavy machinery.

It's deliberately concentrated on a singular pedestrian island in the middle of the busy intersection, and only the most observant of passers-by will notice it. Rising up from what seems to be beneath the street, the hum was originally an art installation by Max Neuhaus, created in 1977 without signage or notification essentially to see if anyone on the world's busiest street would notice.

The strange, unavoidable, barely noticeable humming noise is now heard by hundreds of people each day with only a minute portion of them actually regsitering it.