G-Cans Project

"The Metropolitan Area Outer Underground Discharge Channel (首都圏外郭放水路 shutoken gaikaku hōsuiro), also known as the G-Cans Project, is an underground water infrastructure project in Kasukabe, Saitama, Japan built for preventing overflow of the city's major waterways and rivers during rain and typhoon seasons."

This looks like abou the coolest place ever, just throw me down there with a ton of paint and some beers and i'll be happy for the rest of my life. It's open to the public too, bonus.