The G-Cans project, more officially known as the Metropolitan Area Outer Underground Discharge Channel, is a massive underground waterway and water storage area built to protect Tokyo from flooding during monsoons.
G-Cans opened in 2009 after 27 years of construction. With its 59 pillars, miles of tunnels, and 83-foot ceilings, the vast space resembles an underground temple. Five 21-story concrete silos collect rainwater, preventing overflow of the city's rivers and waterways. The humongous drainage system can pump over 12,000 tons of water per minute--the equivalent of four-and-a-half Olympic swimming pools.
There are daily free tours of the drainage system, but you are required to bring your own translator if you can’t understand Japanese. This precaution ensures that, in the event of an emergency, you will be able to follow evacuation instructions.
More photos of G-Cans can be seen on Atlas Obscura.
Wonders of subterranean infrastructure: