What if cities floated on the sea?

Cities are built on land. Some are right next to the sea.

What if a city floated on the water, like a giant raft?

Many of the world's big cities sit right on the coast. Land there can be crowded, and rising seas and storms can flood it.

So here is a thought experiment: what if cities floated on the sea instead? Homes, schools and parks would rise and fall gently with the water. You can find out how floating buildings stay up, what waves and storms would mean for them, and how a floating city could get fresh water to drink.

Floating construction places buildings on buoyant platforms instead of foundations in the ground. Advocates point to crowded coastal land and rising sea levels; the hurdles are structural, environmental and practical.

This thought experiment asks what it would take for whole neighborhoods to float. It looks at the physics of buoyancy, the forces of waves and storms, and the supply of fresh water and other services, using one built floating building and one published proposal as reference points.

Some buildings already float! One office in the Netherlands floats in a harbor.

People also live on houseboats on rivers and lakes.

In Rotterdam, in the Netherlands, Floating Office Rotterdam sits in a former industrial harbor called the Rijnhaven. Dezeen magazine describes it as a three-story building on 15 concrete barges, built with a wooden frame to keep it light enough to float. Pipes in the barges use the harbor water to cool the building in summer and warm it in winter.

In 2022, designers also presented plans for Oceanix Busan, a floating neighborhood off Busan, South Korea, for up to 12,000 people. It is a plan, not a finished city.

Floating Office Rotterdam, designed by Powerhouse Company and home to the Global Center on Adaptation, rests on 15 tension-cabled concrete barges in the Rijnhaven; cross-laminated timber keeps it light, and a pipe system in the barges uses harbor water as a heat exchanger (Dezeen, 2022).

Oceanix Busan was presented at a UN-Habitat roundtable in April 2022: three linked platforms for housing, public space and research, for up to 12,000 people, anchored with removable pile moorings and connected to land by bridges (NBC News). At the time, completion was described as unlikely before 2025; this lab treats it as a proposal, not a built city.

Let's pretend a whole town floats on the sea.

It bobs up and down gently. When the sea rises, the town rises too!

Imagine a town on big floating platforms, joined by bridges, with homes, gardens and bike paths and no cars. When the sea level rises, the whole town rises with it.

But a floating city would face big questions. It would need to stay afloat however many people moved in. It would need shelter from big waves and storms. And it would need fresh water, because seawater is far too salty to drink. Engineers are working on each problem, and not all have been solved at city size.

Oceanix's designers said their Busan platforms should withstand 100-year storms and rise with sea level, with buildings of no more than five stories in timber and bamboo, and no conventional cars (NBC News, 2022). These are design intentions, not demonstrated performance.

Scaling up raises open questions: load and stability as populations change, mooring forces in storms, wave exposure outside sheltered water, and utilities such as fresh water, energy and waste. The thought experiment treats each as a design problem with trade-offs rather than a solved one.

Water pushes up on things that sit in it. That push is called buoyancy.

If the push is as big as the thing's weight, it floats!

Even huge, heavy things can float. When something sits in water, it pushes some water out of the way, and the water pushes back up. The OpenStax physics textbook explains that this upward push, the buoyant force, equals the weight of the water pushed aside. That's Archimedes' principle.

If an object can push aside water weighing as much as itself before it sinks too deep, it floats. That's why a floating building rests on wide, hollow barges and is built from light materials like wood.

Archimedes' principle: the buoyant force on an object equals the weight of fluid it displaces (OpenStax §11.7). An object floats when it can displace its own weight of water; hollow, wide hulls or pontoons achieve this with modest draft.

For a floating building, every added load, people, furniture, stored water, increases draft, so designers limit weight, as Floating Office Rotterdam's timber structure shows, and size pontoons with margin. Stability against tipping, wave loads and long-term durability in salt water are separate engineering problems that buoyancy alone does not solve.

Big storms make big waves far out at sea. Calm harbors are safer.

Seawater is too salty to drink. Machines can take the salt out, but it uses lots of energy.

Waves grow bigger when wind blows over a long stretch of water, so the open ocean in a storm can have huge waves, while a harbor or lagoon stays calmer. That's one reason today's floating buildings sit in sheltered water.

Fresh water is another puzzle. The U.S. Geological Survey says seawater contains about 35,000 parts per million of salt, while fresh water has less than 1,000. Removing the salt, called desalination, works, but the USGS says it's expensive, uses lots of energy and needs large facilities.

Wave height grows with wind speed, duration and fetch; hurricane winds above 120 km/h over fetches greater than 300 km can generate large, destructive storm waves (University of Hawaiʻi). Sheltered harbors and lagoons limit fetch, which is why built floating structures favor them.

USGS: seawater averages about 35,000 ppm dissolved salts, fresh water less than 1,000 ppm, and current desalination processes are expensive, energy-intensive and large-scale. North America's largest plant, at Carlsbad, California, supplies 50 million gallons a day, at about twice the cost of other water sources.