Back in September, the Obayashi Corporation announced it was working on a project to build a 96,000km-long elevator into space. Now another Japanese tech company has entered the race for the most outlandish construction project ever with plans to build an undersea city.

Shimizu Corporation's Ocean Spiral, as it's called, will take around five years to complete, cost nearly £17 million and house 5,000 people. Engineers at the company believe that the technology to build the city will be available in around 15 years' time.

Blueprints for the ambitious project were unveiled in Tokyo this week. It features a spherical dome which floats on the surface of the sea and retreats underwater in the event of storms. The corporation wants to loan the dome out for commercial use such as office space, hotels and apartments. A corkscrew-shaped structure runs from the dome to the seabed, mining precious rare minerals and producing methane to power the eco-friendly city.

Shimizu spokesman Hideo Imamura told the Guardian: "This is a real goal, this is not a pipe dream." The corporation is not definitely not short on ambition: a glance at their "dreampage" of "proposals to benefit future generations" reveals plans to construct floating cities, a space hotel and a ring around the moon. 

How crazy is the idea of an underwater city? Well, we did land on a comet this month. If nothing else, the runaway box office success of Interstellar shows that the film's depiction of a resource-starved population – and its subsequent spaceward search for alternative means of living – definitely hits a little too close to home. 

But Tokyo University researcher Christian Dimmer sounded a note of caution over handing our future to businesses like the Shimizu Corporation.

"We had this in Japan in the 1980s, when the same corporations were proposing underground and 'swimming' cities and 1km-high towers as part of the rush to development during the height of the bubble economy," he said.

"It’s good that many creative minds are picking their brains as to how to deal with climate change, rising sea levels and the creation of resilient societies – but I hope we don’t forget to think about more open and democratic urban futures in which citizens can take an active role in their creation, rather than being mere passengers in a corporation’s sealed vision of utopia."

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