BLDGBLOG: Tokyo Secret City

(...) there are “seven riddles” about this underground world, a secret Subtokyo of tunnels; the parallel subways were only mystery number one: “The second reveals a secret underground complex between Kokkai-gijidomae and the prime minister's residence. A prewar map (riddle No. 3) shows the Diet in a huge empty space surrounded by paddy fields: “What was the military covering up?” New maps (No. 4) are full of inconsistencies: “People are still trying to hide things.” The postwar General Headquarters (No. 5) was a most mysterious place. Eidan’s records of the construction of the Hibiya Line (No. 6) are hazy to say the least. As for the “new” O-Edo Line (No. 7), “that existed already.” Which begs the question, where did all the money go allocated for the tunneling?”

Shun even claims to have uncovered a secret code that links a complex network of tunnels unknown to the general public. “Every city with a historic subterranean transport system has secrets,” he says. “In London, for example, some lines are near the surface and others very deep, for no obvious reason.”
—BLDGBLOG: Tokyo Secret City