Japan Is Building a Futuristic Underwater City, and It’s Powered by the Temperature of the Ocean Itself.

As the effects of climate change accelerate, the question of where and how humanity will live in the coming decades is no longer hypothetical. Rising sea levels, extreme weather events, and dwindling land resources are forcing a global reconsideration of urban planning and infrastructure. In response to these mounting pressures, Japan has unveiled a vision that is as ambitious as it is unconventional: Ocean Spiral, a proposed underwater city powered entirely by the sea itself. Developed by Shimizu Corporation in collaboration with academic and industrial partners, this submerged metropolis would harness ocean thermal energy, recycle carbon, and produce fresh water—all while housing thousands of people beneath the surface of the ocean.

With many coastal regions facing increasing vulnerability, and entire island nations under existential threat, Ocean Spiral offers a radical yet technologically plausible rethinking of sustainable living. But as with any large-scale innovation, it also raises questions about cost, equity, and the balance between visionary design and practical resilience. Through a closer examination of the project’s engineering, motivations, and critiques, we can gain a deeper understanding of what it represents, not just as a blueprint for future cities, but as a reflection of how humanity might adapt to a planet in flux.

A Radical Vision Beneath the Waves

Japan’s Ocean Spiral project is a bold reimagining of urban life, proposing a self-sustaining underwater city that merges architectural innovation with environmental foresight. Conceived by Shimizu Corporation—a company known for its visionary proposals, including lunar bases and floating botanical cities—Ocean Spiral aims to house up to 5,000 people in a futuristic habitat built beneath the ocean’s surface. At its core is a large sphere situated just below the waterline, designed to contain residential areas, commercial spaces, and hotels. From this sphere, a spiral structure extends 15 kilometers downward to the ocean floor, forming the backbone of a city that is not only habitable but potentially sustainable and energy self-sufficient.

The spiral itself is more than a structural feature—it’s a conduit for innovation. At the deepest point, approximately 4,000 meters below sea level, lies what Shimizu calls the “eco-friendly earth factory.” This facility is designed to use marine microorganisms to convert carbon dioxide into methane, while the spiral structure would also host energy-generation systems that harness the ocean’s thermal gradients. This vertical integration of living, resource production, and energy generation turns the city into a closed-loop ecosystem, eliminating the need for extensive surface infrastructure. The design reflects an ambitious shift: rather than conquering nature or merely adapting to it, Ocean Spiral proposes to integrate human civilization within it.

Collaboration is essential to bringing this vision to life. Shimizu is working alongside researchers from Tokyo University and energy companies across Japan to ensure the project’s feasibility, with a long-term plan to attract private investment and government backing. Although construction is still over a decade away, estimated to be viable in 15 years, the company maintains that the necessary technologies are within reach. According to spokesperson Hideo Imamura, the concept is not science fiction, but a tangible goal, much like how mobile phones were once considered improbable. With Ocean Spiral, Shimizu is not just proposing a new kind of city. It is challenging conventional ideas about where and how we live.

Rising Seas and Sinking Options

At the heart of Ocean Spiral’s audacious design lies a pressing global challenge: the escalating impact of climate change on coastal and island communities. Rising sea levels, driven by melting polar ice and thermal expansion of seawater, are already beginning to engulf low-lying nations and threaten major cities around the world. According to the Intergovernmental Panel on Climate Change (IPCC), global sea levels have risen by more than 20 centimeters since 1900. They could increase by as much as one meter by the end of this century if current emissions trends continue. For the world’s 52 small island nations—and even for Japan, with its densely populated coastal regions and history of devastating tsunamis—this is not a distant scenario but an unfolding reality that demands urgent adaptation.

Shimizu Corporation presents Ocean Spiral as a forward-thinking response to this environmental threat, an emergency blueprint for habitation in an increasingly unstable world. Rather than reinforcing the shoreline or retreating from the coasts, Ocean Spiral proposes a third option: embracing the sea as a space not just for survival but for thriving. In this underwater city, elevation becomes irrelevant, natural disasters like typhoons and floods lose their destructive leverage, and sea-level rise ceases to be an existential concern. The design envisions a city that is immune to many of the vulnerabilities that plague terrestrial infrastructure, providing not just safety but long-term resilience in the face of escalating climate volatility.

This vision also reflects a broader reckoning within architecture and urban planning, where the boundaries between built environments and natural ecosystems are being re-evaluated. Projects like Ocean Spiral suggest a pivot away from conventional, land-based expansion toward more radical, adaptive, and environmentally integrated models of habitation. In this context, the ocean is no longer viewed as an obstacle to be contained, but rather as a frontier for innovation. While the scale of such projects naturally invites skepticism, their conceptual value lies in pushing the limits of what future cities could look like in a warming world. For Japan, a country deeply aware of its seismic risks and ecological vulnerabilities, the development of such alternative habitats may prove to be a necessity.

Powering a City from the Sea

One of the most ambitious aspects of the Ocean Spiral concept is its plan to generate power, process water, and sustain human life using only the resources found in the surrounding ocean. At the center of this vision is a relatively underutilized yet scientifically sound energy source known as Ocean Thermal Energy Conversion (OTEC). This technology leverages the natural temperature difference between sun-warmed surface water and the cold, deep-sea water thousands of meters below. By circulating a working fluid with a low boiling point, typically ammonia or a similar compound, between these temperature layers, OTEC systems can drive turbines to produce electricity in a clean and continuous manner. Unlike solar or wind energy, which are variable by nature, OTEC offers the promise of round-the-clock power as long as the temperature differential is maintained, which it is in most tropical and subtropical waters.

In Ocean Spiral’s design, power generators embedded along the descending spiral structure would continuously harvest this energy as they bridge the thermal layers of the ocean. This integration of infrastructure and environment is not just efficient but spatially elegant, transforming a single vertical structure into a power plant, a foundation, and a utility corridor. Complementing this energy system would be desalination facilities that use hydraulic pressure to convert seawater into drinkable freshwater, directly addressing another major constraint in many coastal and island regions. Additionally, the project includes plans for bioreactors at its deepest level, where marine microorganisms would be used to convert captured carbon dioxide into methane, a form of clean fuel that could either be used on-site or stored for future applications. If realized, these systems would make the city largely self-reliant in terms of energy, water, and even basic fuels.

What sets Ocean Spiral apart from traditional sustainability initiatives is the way it fuses these technologies into a cohesive, purpose-built ecosystem rather than retrofitting them onto existing structures. Its design is not simply about reducing environmental impact but about redefining the relationship between humans and the ocean, using science and engineering to establish a new kind of symbiosis. While many of these technologies already exist in experimental or pilot-scale forms, scaling them to support a full human settlement beneath the waves would be unprecedented. Yet this is precisely what Shimizu Corporation and its collaborators are setting out to explore, not as a distant fantasy but as a real-world alternative to the increasingly precarious state of land-based urbanization.

Between Vision and Viability

As compelling as the Ocean Spiral’s technical blueprint may be, translating such an audacious vision into reality inevitably raises questions of feasibility, both financial and social. Shimizu Corporation estimates the total cost of constructing the underwater city at $25 billion, a figure that, while staggering, is not inconceivable when compared to the budgets of major infrastructure megaprojects like international airports or high-speed rail systems. However, funding such a project would require a complex coalition of private investors, government subsidies, and technological stakeholders—none of which are guaranteed. While the company has secured academic partnerships with institutions such as Tokyo University and is in discussions with Japanese energy firms, the actual commitment of capital and political will remains a significant hurdle. According to Shimizu, construction could begin within 15 years; however, this timeline depends heavily on continued technological advancements and regulatory support.

Beyond economics, Ocean Spiral has sparked debate over the kind of future it represents—and for whom. Critics argue that such large-scale, high-tech urban environments risk becoming exclusive havens for the wealthy, echoing dystopian themes from films like Elysium, where the privileged escape environmental collapse while others are left behind. Christian Dimmer, an urban engineering expert at Tokyo University, has voiced concern that these projects tend to reflect corporate-controlled, sealed-off utopias rather than participatory, democratic visions of the future. While he acknowledges the creative ambition behind Ocean Spiral, he warns against allowing innovation to outpace inclusivity. In his view, cities of the future should not only be resilient and technologically advanced but also socially equitable and open to citizen involvement in their design and governance.

The project also faces skepticism due to its conceptual lineage. Similar underwater and floating city ideas have circulated since at least the 1980s, often presented at expos or in speculative design showcases, only to remain unrealized due to insurmountable logistical barriers. Critics note that Shimizu’s portfolio—though rich with ingenuity—includes other futuristic proposals like space elevators and lunar bases that have yet to progress beyond paper. However, Ocean Spiral differs in that it responds directly to tangible environmental threats and employs emerging technologies that are already in experimental use. Whether this will be enough to move it from concept to construction remains to be seen, but the project’s existence is already shaping conversations about what future resilience might look like—and challenging conventional limits on where cities can be built.

Rethinking Where We Belong

Whether or not the Ocean Spiral is ever constructed, its true impact may lie in the questions it forces us to confront. As rising seas and environmental instability render traditional models of urban development increasingly untenable, the need to reimagine where—and how—we live becomes more than speculative fiction; it becomes a survival imperative. Projects like Ocean Spiral compel architects, policymakers, and citizens alike to stretch their thinking beyond the surface, both literally and figuratively. They push the boundaries of engineering while also challenging deeply rooted assumptions about the separateness of human life from the ocean, the land, and the broader ecosystems we inhabit.

In proposing an underwater city powered by the sea itself, Shimizu Corporation is not simply designing a novel place to live—it is advocating for a new kind of relationship between civilization and nature. One where infrastructure adapts to the environment rather than the other way around, and where resilience is built not through isolation from ecological forces, but through deeper integration with them. This approach echoes emerging philosophies in urban sustainability that prioritize regenerative systems, circular economies, and climate adaptation over expansion and extraction. If pursued ethically and inclusively, such ideas could lead to more just and ecologically sound futures—whether below the waves or above them.

Yet innovation without reflection can risk reinforcing existing inequalities or sidelining more immediate, accessible solutions. As we imagine radical futures, we must also ensure they are grounded in democratic values, inclusive design, and a commitment to equity. The Ocean Spiral may remain, for now, a conceptual blueprint, but its value lies in how it provokes action and dialogue. It reminds us that the most important question is not just can we build such cities, but should we—and for whose benefit? As climate pressures escalate, the answers we offer will help shape not only our physical landscapes but also the ethical contours of our shared future.