On July 10th, 1975, the NY Times printed a photo showing visitors at the a minipark behind the McGraw-Hill Building, "emerging from a 17 1/2-foot-long glass tunnel that runs beneath a waterfall at site." 40 Years later and that waterfall is still there, partially housed in plexiglass at 1221 6th Avenue, between 48th and 49th Streets. While walking through the tunnel won't cool you down, you can attempt to get some backsplash if you lean in close enough at certain areas.
Check out the urban oasis, which isn't too far from Times Square... and on some days, during the lunch rush, can get almost as crowded. Just like back in 1978, when the Times ran a photo spread showing the "charming" park filling up on a hot summer day.
The water feature itself is about 40-feet-wide, and 20-feet-tall.

(Photo by Ben Miller/Gothamist)
[h/t Untapped Cities]