JAPAN - From the world's largest floodwater diversion facility to bridge tunnels and massive oil storage tanks across the country, Japan has done them all, using its underground expertise gained from building its extensive subway networks.
Since the first subway line in Tokyo - and in Asia as well - began operations in 1927, Japan's labyrinthine underground network has dug deep. There are now more than 40 subway lines covering the country's major urban centres and accounting for more than five billion rides a year.
Major train stations also anchor some of the country's famous chikagai, or underground shopping streets, providing both the infrastructure and the shopper numbers.
The iconic Tokyo Station, which has an underground space of more than 70,000 sq m and sees more than 400,000 passengers daily, has one of Japan's largest chikagai, boasting more than 100 shops selling souvenirs, ramen, character goods and clothes, as well as restaurants and cafes.
But such space-saving underground malls are not located only in densely populated Tokyo. Prefectures such as Hokkaido and Okayama also have chikagai which are better described as underground shopping malls.
As Mr Yoshio Sumikawa, head of the association for the promotion of shops in the Shibuya chikagai, put it, it is more about the convenience rather than a need to build underground. "Regardless of what weather it is, people can shop here in comfort," he said.
Some underground projects, however, are crisis-driven.
As Japan is typhoon-prone, to protect Tokyo and other cities from floods, the government has built massive anti-flood underground facilities, including the Metropolitan Area Outer Underground Discharge Channel, which channels water from Tokyo's rivers through a tunnel 6.5km long and 50m deep to the Edogawa River in neighbouring Saitama prefecture.
The 1973 oil crisis led to the building of installations across Japan containing oil storage tanks with a capacity of 50 million kilolitres of crude oil.
Also found underground are bicycle parks, telecommunication cables, and even a library.
While some construction companies have shared their visions of futuristic cities that would allow millions to live, work and play 150m below ground, these are still at the conceptual stage.
The bread-and-butter of Japan's underground expansion is still linked to its subway system.
Sendai's new Tozai line is expected to start operations in 2015, and Shinjuku and Shibuya stations are undergoing revamps that would increase their already extensive underground space.
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