REACH: New York

Camouflaged as an only slightly odd portion of New York City's 34th Street subway station, is an unusual pair of musical instruments.

You probably won't see them unless you know where to look. There's one on each N/R train platform, opposite each other on the uptown and downtown tracks. The installation/instruments appear to be simply green metal bars—a sort of surplus municipal green, of the kind easily mistaken for subway infrastructure—mounted a foot or two above eye level. At regular intervals along the bars are small holes, and between the holes are what look like air ducts.

But the air ducts are really speakers, and the holes actually contain motion sensors. Reach a hand up in front of one of the holes, and you'll trigger a sound. You can move from sensor to sensor, playing a different sound on the instrument as you wait for your train. 

The installation, called REACH: New York, was built by Christopher Janney in 1995. Janney updates the sounds every year. In 2015, the selection included a number of traditional musical instrument sounds along with six kinds of frog sounds: Eastern Cricket Frog, American Toad, Great Plains Toad, Canadian Toad, Boreal Toad, and Blanchard's Cricket Frog. The 2016 update features marimba and xylophone tones.  In early 2020, the sounds are bird calls.

There are no big signs announcing its presence and watching commuters accidentally discover their existence is one of the secret joys of the city. 

Update as of January 2020: REACH: New York now plays bird calls.