If you think the urban sprawl of Tokyo looks impressive from above, wait until you factor in the areas below ground. With demand for commercial and residential space increasing on the surface, more and more developers are exploring ways to utilize space underground.

Hundreds of curious sightseers attended a preview of Tokyo’s latest underground passageway — a 900-meter tunnel in Minato Ward connecting Shimbashi to Toranomon — on March 23. More than 38,000 vehicles are expected use the Tsukiji-Toranomon Tunnel every day, bypassing one of the capital’s busiest business quarters and alleviating traffic congestion above ground.

Building the four-lane road, dubbed “MacArthur Road” after U.S. Gen. Douglas MacArthur, who led the Allied Occupation of Japan after World War II, certainly hasn’t been easy. Construction work was originally slated to start in 1946 but the project fell behind schedule after running into delays related to land-use planning. Indeed, structures that already exist underground forced developers to take the road right through Toranomon Hills skyscraper’s basement floor. “We wanted to preserve the surface of the area as much as we could, since there are locals who hold a strong attachment to the district,” says Takeshi Oyagi, chief of the Tokyo Metropolitan Government’s Urban Development Office, adding that it was “not technically difficult” to run a road through the basement of a 52-story building.

The most difficult aspect of the project was finding the space to dig the road. The metropolitan government says Tokyo has approximately 63,000 underground areas, with underground paths, subway systems and shopping complexes comprising 40 percent of the total. The eight largest underground shopping areas in the capital add up to about 214,000 sq. meters — about the same as having five Tokyo Domes buried underground.

The Tsukiji-Toranomon Tunnel is buried about 2 meters underground, and is separated by just 30 cm — roughly the width of a Japan Times On Sunday page — from an underground utility conduit jointly operated by gas, water and telecommunications companies, among others, that runs beneath it.

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The conduit, meanwhile, is also located 30 cm above the Toei Mita Subway Line that connects Uchisaiwaicho and Onarimon stations. If the Tsukiji-Toranomon Tunnel is extended south toward the waterfront in the next few years (as scheduled), it will also require the route to be squeezed into a space that would leave it only 2 meters from the Toei Asakusa Line below.

The Tsukiji-Toranomon Tunnel was created “with the utmost care and attention,” says Masashi Sugaya, an official at the metropolitan government’s Construction Bureau. An open promenade lined with trees will be created over the underground path, where Tokyo says it hopes to replicate the feel of the famed Avenue des Champs-Elysees in Paris.

Beneath the streets, the congestion is likely to become even worse in future. News reports say that Tokyo Metro might even try to squeeze another underground station into the area, between Kasumigaseki and Kamiyacho stations on the Hibiya Line.

Tokyo’s development underground kicked off in 1927 with the opening of what is today the Tokyo Metro Ginza Line, which initially connected Asakusa Station to Ueno. By 1930, the first underground shops had opened around Ueno Station.

Today, Tokyo Metro operates 179 stations, which carry more than 6 million passengers every day. In addition, the metropolitan government manages 106 stations, which carry more than 2 million people on a daily basis. The subway lines dodge, dip and rise above each other underground, while also avoiding other facilities such as electricity, gas and sewage pipes, as well as communications cables and underground parking lots.

Massive underground stations such as Shinjuku are lucky if they have 30 cm of space between any two facilities. The Fukutoshin Line, which began operation in 2008, travels under the Marunouchi Line and over the Toei Shinjuku Line routes at Shinjuku-Sanchome Station. Just 11 cm separates the Fukutoshin Line and the Shinjuku Line that runs beneath it.

“Obviously, Tokyo’s underground is becoming congested,” says Taro Kasuya, chief researcher at the Urban Underground Space Center of Japan.

“Some of the earlier development projects weren’t properly planned out,” Kasuya says, adding that little thought seems to have been given to future traffic below ground.

One example is the Ginza Line, which was built at a relatively shallow depth, taking full advantage of its first-come, first-served footing. The lowest station along the subway line is Nihombashi, located about 10 meters below ground.

But once demand to create more subway lines increased as Tokyo expanded, developers had no other choice than to dig deeper. There was no way to build additional lines unless they avoided existing ones.

The lack of development planning increased costs and delayed plans to build later subway lines or underground paths. The deepest station on the Toei Oedo Line, which began full operation in 2000, is Roppongi Station, which is 42 meters below ground.

The ant nest of subway lines beneath Tokyo appears to have reached its limit, with the fragmentation of underground land making it impossible to launch extensive new projects.

However, Kasuya at Urban Underground Space Center of Japan believes the space below ground is one of humanity’s last frontiers.

“We’ve constructed the (634-meter-tall) Tokyo Skytree above ground. In contrast, however, there hasn’t been much use of underground space beyond 50 meters,” he says. “There is still room for expansion.”

Kasuya’s organization, which consists of academics, technicians and those involved in urban development from both the public and private sectors, has been calling for the use of underground space to be diversified. For example, it has called for transportation systems that don’t need to be located near the surface to be moved deeper underground, thereby leaving shallow areas open for redevelopment.

Pipes carrying sewage and water can also be placed at deeper levels, Kasuya says, adding that this would allow underground paths and arcades to be constructed at more accessible depths.

“This could help introduce major changes to the way goods are distributed in Tokyo,” Kasuya says, pointing to a plan that would see shipments carried to the center of the city via an underground thoroughfare direct from Tokyo Bay.

“This would ease traffic congestion in the city, cause fewer accidents and allow us to control the amount of exhaust gas that is released into the atmosphere,” he says.

On Aug. 16, 1980, at about 9:30 a.m., a small gas explosion occurred in an underground shopping district connected to JR Shizuoka Station. As firefighters gathered in the area, a second explosion occurred. The second blast was caused by sparks that ignited subsequent gas leaks that, without proper exhaust ports, had accumulated in the underground arcade. Fifteen people were killed by the blast, with more than 200 others injured.

In the wake of the explosion, the government revised laws on underground business areas, tightening regulations on the installation of firefighting equipment as well as systems that shut down to contain the spread of leaking gas.

Whenever there has been an accident in an underground facility, the government has responded by tightening regulations. Indeed, underground facilities now have to follow rules on just about everything — there’s even a regulation that specifies the number of exits a facility must have.

“Underground facilities are safe these days,” Kasuya says. “It is like riding in a submarine.”

Kasuya says many underground facilities in Japan are actually safer than structures on the ground, adding that earthquakes are less powerful the further down you are.

The Tsukiji-Toranomon Tunnel, enforced with multiple quake-resistant support beams, can withstand temblors on par with the Great Kanto Earthquake of 1923 and the Great Hanshin Earthquake of 1995. The tunnel has also been positioned to avoid damage in the event of a quake generating the highest possible tsunami in Tokyo Bay, the metropolitan government says.

Confident that space under Tokyo can be better utilized, the government has shown signs it is finally ready to tackle the problem. With space in the capital becoming difficult to find, the Diet passed a new law in 2001 on the use of daishindo (extreme underground), or the areas located below 40 meters.

Under the law, developers seeking to create public-service grids such as roads, subways and sewage systems can to some extent work freely under that depth in certain areas in Kanto, Chubu and Kinki. Such areas are described as “a valuable space” on the land ministry’s website. Developers can also skip negotiations with landowners and construct a route that requires the shortest distance.

Developers have already taken advantage of the 40-meter rule, and a new water-pipe system in Kobe is scheduled to be completed next year. Because it is located more than 40 meters below ground, construction costs have been cut by ¥2 billion and the project is expected to be finished five months ahead of schedule.

Asked which part of Tokyo’s underground has the most potential for growth in future, Kasuya says Otemachi’s Yaesu area is the strongest candidate.

“It’s crowded with tiny buildings but has a lot of potential as a tourist spot,” Kasuya says. By utilizing the daishindo rule and taking advantage of the upcoming Olympic Games, he believes “the area is ripe for some dynamic development projects.”

Ever tried to leave Otemachi without an umbrella and found it’s raining outside? The best thing to do in such situations is head deeper underground, says Taro Kasuya of the Urban Underground Space Center of Japan. The area’s underground pathways are so well-connected that one can walk all the way from Otemachi, cross Tokyo Station and the Yaesu shopping district, past JR Yurakucho Station and make it as far as Kabukiza theater in Higashi-ginza without ever needing to walk on the surface.

All in all, that’s a walk of 4.05 km.

All in all, that’s a walk of 4.05 km. Shinjuku isn’t far behind Otemachi in terms of its underground connectivity. One can start at Nishi-Shinjuku Station, walk past the Tokyo Metropolitan Government building, work your way through Shinjuku Station and arrive at the front doors of Takashimaya Department Store south of the station. That’s an underground walk of 2.75 km.

Approximately 86 percent of the new maglev bullet train line, which is scheduled to begin commercial service between Tokyo and Nagoya in 2027, will run underground. Its base terminal is scheduled to be constructed in Shinagawa, approximately 40 meters below the surface.

Tokyo Skytree is 634 meters high. The deepest vertical pile supporting the tower reaches 50 meters below ground.

The transmission of power using buried electricity cables often causes overheating problems underground. However, the invention of new superconductive cables, which use liquid nitrogen as coolants, will make it easier to move power transmission lines from above ground to below the surface.

Five subway lines are connected to Otemachi Station — the most in Tokyo. The underground facilities around Otemachi are so expansive that station employees use bicycles to check that all exit gates have been shut properly once services stop for the evening.

Underground construction work beneath Tokyo often unveils the city’s past, which gives us a glimpse of what the megalopolis once looked like. For example, workers uncovered fossils of the extinct Naumann elephants during the construction of Toei Shinjuku Line’s Hamacho Station. Remains of stone walls of Edo Castle, artifacts from the Neolithic Jomon Era and oval coins from the Edo Period have also been dug up around the city.

Artifacts uncovered during the digging of the Tsukiji-Toranomon Tunnel were mainly from the Edo Period, most of them used by locals in their daily lives. They included cooking wares, rice bowls, hand mirrors and sake bottles inscribed with the word “Tamura-cho” — the name of the district in those days.