Yesterday, an Untapped Cities reader contacted us asking us what those mysterious green mailboxes are on the streets of New York City. The mailboxes are known as relay boxes in New York City. Luckily for all, our favorite book The Works: Anatomy of a City by Kate Ascher has a little section on it. Asher writes in “Relaying the Mail”:

Familiar destinations to most New Yorkers, over 8,000 blue mail collection boxes are spread out across the city. Less familiar are the olive green ‘relay boxes,’ which are used a temporary holding spots for letter carriers whose carts or mail sacks cannot hold all the mail to be delivered within their territory. Presorted mail is delivered by postal van to the relay boxes at appropriate points during the day for pickup by letter carriers when they are ready for the next load.

Digging into it, we saw that Gothamist also posed the same question last year and even contacted the USPS, whose rep said the function of the boxes was  “proprietary.” Why they would say that when it’s out there in a book you can buy is beyond us but here you go! In other fun news, did you know mail used to be sent by pneumatic tube underground in NYC and that you can still find remnants of this network?

Read more from our Cities 101 series about how stuff works in the city. And yes, we’re on a big of a mailbox kick today with the little US Mail box found 9 feet up in a tree at Union Square…