Stumbled upon a nuclear war fallout shelter! During the Cold War, the US took things seriously and actively built shelters in the 1960s to prepare for a Soviet attack. In New York alone, they planned to build enough shelters to accommodate over 11 million people (!). Looks like they didn't manage to build them all, but there are still thousands scattered throughout the city. And get this—until the mid-1970s, they actually maintained fresh food supplies in the already-built shelters. Then times changed, but the bunkers stayed put.

Can't say I spot them everywhere. I mean, I've only come across a couple. But apparently, nowadays a lot of them have just had their markings removed.