underground at Kasimigaseki
This may be a secret underground route exist within Kasumigaseki station, which is the closest station from the Diet Building of Japan.

Well, this is actually a passage way between Chiyoda-line and Marunouchi line in Kasumigaseki, but has some mysterious feeling.

I am quite sure that there are many private underground passages for communicating different governmental buildings and for escaping within this Kasumigaseki area.