Forget colonies in space, one Japanese construction company says in the future human beings could live in huge complexes that corkscrew deep into the ocean.

Blue sky thinkers say around 5,000 people could live and work in a modern-day Atlantis, a sphere 500 metres (1,500 feet) in diameter that houses hotels, residential spaces and commercial complexes.

The vast globe would float at the surface of the sea, but could be submerged in bad weather, down the centre of a gigantic spiral structure that plunges to depths of up to 4,000 metres.

The spiral would form a 15 kilometre (nine mile) path to a building on the ocean floor, which could serve as a resource development factory that could collect rare metals and rare earths.

Visionaries at Shimizu, a construction company, even think they could use micro organisms called "methanogens" to convert carbon dioxide captured at the surface into methane.

The sci-fi concept is the work of several organisations, including Tokyo University and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

It envisages using the wide difference in water temperatures between the top and the bottom of the ocean for generating power.

Shimizu says the Ocean Spiral would cost three trillion yen ($25 billion), and all the technology could be in place by 2030.

It is the third such project unveiled by the company after a floating metropolis and solar power ring around the moon.

"The company in cooperation with many organisations has spent two years to design the project working with technologies we think will be plausible in the future," said a Shimizu spokesman.

In 2012, another Japanese construction firm, Obayashi Corp, claimed it could execute an out-of-this-world plan to put tourists in space within 40 years by building an elevator that stretches a quarter of the way to the moon.

Obayashi claimed it could use carbon nanotube technology, which is more than 20 times stronger than steel, to build a lift shaft 96,000 kilometres (roughly 60,000 miles) above the Earth.

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Shimizu

Obayashi