For 56 years, beneath the streets of Manhattan and Brooklyn, brass cylinders the size of small artillery shells screamed through cast-iron tubes at 30 miles per hour, carrying roughly 97,000 letters per day. The pneumatic tube mail system of New York City was the steampunk internet — a physical packet-switched network where the packets were leather-wrapped canisters and the routers were sweating men in basements pulling levers.

The system opened in 1897 with a piece of theater that tells you everything about Gilded Age optimism. The inaugural canister contained a Bible, a flag, and a copy of the Constitution. The second contained a live black cat. (It survived. Postal workers, perhaps inevitably, made a habit of sending animals through the tubes — including, allegedly, a Siamese cat from a Wall Street office to Brooklyn.)

At its peak the network ran 27 miles of tubes connecting 23 post offices in Manhattan and one in Brooklyn, crossing the Brooklyn Bridge through a tube suspended beneath the roadway. Each canister held about 600 letters and made the trip from the General Post Office to Harlem in roughly 20 minutes — faster than anything that uses the same route today, including the subway.

Here's the part that connects to things you already half-know:

• The system was operated not by the Post Office but by private contractors who charged the government enormous fees — about $17,000 per mile per year. This is the same political fight we have now about ISPs and last-mile internet infrastructure.
• The tubes used 2.5 PSI of compressed air, generated by enormous steam-driven pumps. The same pneumatic technology powered Paris's poste pneumatique, Prague's still-extant system, and the cash-carrier tubes you might remember from old department stores or bank drive-throughs.
• The whole thing was killed not by a better technology but by the automobile. Trucks were cheaper, more flexible, and didn't require the city to dig up streets when a tube needed maintenance. Service ended in 1953.

The truly haunting detail: most of the tubes are still down there. They were never excavated. When you walk through Lower Manhattan, somewhere beneath your feet — between the gas mains and the subway and the abandoned trolley tracks — there are 27 miles of empty cast-iron pipe that once carried Edith Wharton's correspondence at highway speeds. In 2001, USPS briefly studied reactivating the system for inter-office mail before concluding the tubes were too corroded.

The pneumatic mail system is the clearest preview we have of what our fiber-optic infrastructure will look like to people 100 years from now: a once-essential network that became an archaeological curiosity, replaced by something faster but somehow less magical than firing letters through tubes at 30 miles per hour.