What an awesome site! Wish I had time to do some serious spelunking on it. But I happened to hit the West Something Zero Station at random and what didI find? Spies being trained in letterboxing. I love finding real-world applications of it. :)
Isn't it great? There are books on subterranean New York as well. And Paris is another city with lots of fun stuff underground-- any old city (e.g. Rome) or a city with lots of industrious Victorian-era tunneling. But I liked the larger scope of that site, and the attention it pays to disused military holes in the ground.