G-cans (Otherwise known as the Metropolitan Area Outer Underground Discharge Channel) is situated in Saitama.
It's on the Tobu Noda line and he closest station is Minamisakurai. From there it's a 30 minute walk to the complex amid acres of rice fields.
It's my day off today so I stupidly decided it'd be a good time to walk in 35 degree heat but having said that, underground it was a naturally pleasant 15 degrees.
Nothing prepares you for the first sight once you enter the "temple". Photos don't do justice how immense this place is. It's pretty huge. 25.4m high and 177 metres long. The feeling of this place gives you the impression of being on the set of Alien or Indiana Jones.
Unfortunately you only get ten minutes to take photos, and the area to look around is quite restricted. The majority of the time is spent watching a video (complete with cheesy music) explaining how the place works. The tour is all in Japanese, so they say you need someone who can translate for "security" reasons.
This place was used in this Land Rover commercial:
And also this music video by the band um.. Screw.
Oh and don't forget even Pharrell has been down here too. If that isn't enough of a recommend I don't know what is!
The tour is free so money back guarantee! It has to be booked in advance, which you can do from this page (Japanese). For some English information look here.