Shimizu is no stranger to moonshot ideas. It has previously proposed a ring of solar panels around the moon's equator to generate electricity for Earth and a self-sufficient, carbon-negative floating city in the Pacific Ocean. Its most recent idea, however, is more similar to Phil Pauley's Sub-Biosphere 2 self-sustainable underwater habitat.
Shimizu says the basis for the concept is rooted in the huge potential of the deep sea and of the cycles that link in with the air, sea surface, and seafloor. The company outlines five main reasons for developing the project in the deep sea: there is potential for sourcing seafood, producing desalinated water, generating energy, treating carbon dioxide and extracting resources from the ocean and the seafloor.
The Ocean Spiral takes the form of a huge sphere known as the "Blue Garden." It's 500 m (1,640 ft) in diameter, floating for the most part just below the surface, but with its very top breaching the surface. This will contain 75 floors with spaces allocated for hospitality, residential, commercial and research purposes. It is expected to accommodate a population of 5,000, with 4,000 permanent residents and 1,000 visitors.
Access to the Blue Garden and the Ocean Spiral is through the "grand entrance" on the water's surface. The Blue Garden itself contains a huge atrium, hotel suites, an observation gondola, a sea park and a leisure and retail plaza. Temperature, humidity and oxygen levels are controlled inside.
Below the Blue Garden is a spiral that reaches 3-4 km (1.9-2.5 mi) to an "Earth Factory" on the sea-bed. The spiral accommodates the functions required for the Ocean Spiral complex to operate in the deep sea. Power is generated via ocean thermal energy conversion, food is produced via deep sea aquaculture, and water is desalinated. A monitoring facility is also located here and so too a port at which submarines can dock.
The Earth Factory is anchored to the sea-bed and is connected to similar satellite facilities via transport tunnels. These "factories" are used to store, treat and reuse carbon dioxide, as well as to cultivate and develop deep sea resources.
The basis for the construction of the Ocean Spiral is the Blue Garden's sphere shape. This was chosen to provide strength against the external water pressure. The sphere is further reinforced by a central internal tower. A frame for the sphere and its tower will be constructed using a resin concrete. The frame will be covered using triangular acrylic sheets measuring 50 m (164 ft) along the side, and reinforced using semi-transparent fiberglass cross-bracing ribs.
All construction is planned to take place on the surface of the ocean. Shimizu notes that the materials used and processes for construction are likely to be refined as the project moves forward, and through the ongoing development of technology.
The firm reportedly believes the project will cost in the region of ¥3 trillion (US$25.5 billion) and could be completed within 15 years.