Greater Tokyo is the largest city in the world. Home to over 35 million people, the city is built on a floodplain interlaced with more than 100 rivers and it is also one of the densest cities in the world.

This means when Tokyo's crowded urban landscape is hit with torrential rain there is nowhere for the excess water to go, placing the busy metropolis at the substantial risk of flooding. But urban planners have come up with a simple solution to this dilemma: go underground.

Beneath the busy streets of Tokyo, engineers have built an extensive network of 10-metre wide tunnels, a storm water tank able to hold 670,000 cubic metres of water and shafts 70 metres deep.

Join Rachel Mealey in Tokyo, Japan, to see how this "underwater river system" is helping shield the megacity from the threat of flooding. 

Source: Al Jazeera