The flood tunnels of Tokyo are the largest stormwater drain system in the world. A 6.3 km tunnel system connects five water courses to the main river (the Edo) 50 m below the surface. Why is this massive engineering feat necessary? 
Well, more than 100km2 of the Tokyo river basin is below sea level, the average rainfall is 1,530mm and there are eight rivers flowing through the city. Add to this that climate change is making the city experience semi tropical rainfall events even though it’s in a temperate zone and ‘guerrilla rainstorms’ of up to 100mm an hour when the city is only designed to cope with 50mm of rainfall an hour and it is not surprising that Tokyo floods regularly. When the river levels rise they are diverted to the underground storm sewer though shafts that are 30 meters wide and 70m high. At the end of the tunnel is a massive surge tank with pumps that discharge the rainwater at 200m3 per second into the Edo, which can handle the massive volume of water and discharges it within a couple of miles into the sea. Pretty impressive.
For more information check out this video. Or read this article.