[SmellyGeekBoy]

It's amazing just how much stuff there is under Tokyo - not just infrastructure like this and the usual transport links but entire multi-storey shopping malls. There's almost a whole city beneath the city. It's possible to travel for miles without ever seeing daylight!

[ekianjo]

Osaka has also massive undergrounds. Not as large as Tokyo's of course, but massive nonetheless.

[Iv]

My favorite part is the JSDF underground bunkers where they store the giant robots used for disaster relief.

[bamboozled]

Where is this and how do I see it ?

[barry-cotter]

It’s a joke, a mecha robot reference.

[spike021]

Even just the transportation infrastructure is incredible to me. I was just there and it was honestly amazing to me how deep the metro stations can be. Multiple long escalators/stairways from surface to platform.