A Japanese company has come up with a bold and novel solution to the problems facing built-up coastal cities: sink a 500-metre wide sphere underwater that would support a tower filled with homes, stores, offices, a hotel and research facilities.

Shimizu Corporation has been working out the details of its 'Ocean Spiral' city with experts at universities and national agencies, a development that would cost around $26 billion and be constructed in the water.

The entrance would come at 200m below sea level, where sunlight still penetrates, and give way to a spherical 'blue garden' base camp containing a hotel, shops, convention facilities, apartments, offices, research facilities and laboratories.

(Shimizu Corporation)

(Shimizu Corporation)

(Shimizu Corporation)

Citizens would also be able to travel deeper into the ocean, departing from the 'deep sea gondola lobby' through an 'infra spiral'.

The city, first proposed in September, would be sustainable and almost entirely self-sufficient, with thermal power being generated using the temperature difference between surface water and the deep sea.

"Approximately 70% of the earth’s surface is covered by the ocean, and the ocean itself consists of approximately 80% deep sea," Shimizu Corperation wrote alongside new concept art it shared for the city.

"The deep sea offers enormous potential for ensuring effective and appropriate cycles and processes in the earth’s biosphere."

(Shimizu Corporation)

(Shimizu Corporation)

(Shimizu Corporation)

(Shimizu Corporation)

(Shimizu Corporation)

"Unfortunately, we have yet to make the most of this potential. This large-scale concept seeks to take advantage of the limitless possibilities of the deep sea by linking together vertically the air, sea surface, deep sea, and sea floor.

"Now is the time for us to create a new interface with the deep sea, the earth’s final frontier.

"Breaking free from past patterns of land development, which have focused mainly on efficiency, this plan is intended to promote true sustainability while maximizing use of the deep sea’s resources."

Ocean Spiral is designed to relieve pressure for more housing on land and offers "unlimited potential for treating CO2 emissions".