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Extending south of China and ringed in by the Philippines, Vietnam, Brunei, and Malaysia is a 1.35 million-square-mile body of water known as the South China Sea. If it truly is the case that East Asia is the global economy’s center of gravity, then the South China Sea might be its singularity. In 1405, the Chinese admiral Zheng He set sail with a fleet of treasure ships, traveling to neighboring nations and eventually as far as Mombasa, spreading knowledge of China’s wealth. Today, the South China Sea is again a venue for China to display its power — though with a very different fleet.

More: China’s new heavy-lift ship is one of the biggest ever built

Although from on high the sea may seem a blue wasteland, punctuated occasionally by specks of uninhabited land, the islands have seen a frenzy of activity in recent years: China has constructed a series of artificial islands throughout the area. These artificial islands are a showcase of Chinese engineering, and this muscle-flexing has provoked strong reactions from China’s neighbors in the region, particularly the Philippines, which brought a suit against China to the Permanent Court of Arbitration in The Hague. On July 12, 2016, the international tribunal ruled against China, although the superpower refused to acknowledge the decision or even the court’s jurisdiction.

What exactly are China’s artificial islands, and why are they so important? As it turns out, China’s island-building plan sits on a contentious intersection of technology, politics, and the environment.

How do you build an island?

For those wondering what an artificial island is made of, the answer is the same thing most islands are made of: sand. The process for building these islands is remarkably simple, although the technology involved is imposing.

The first requirement for an island is a base to build on. Naturally formed islands don’t float in the water; rather, an island is simply the top, visible part of a land mass that is mostly underwater.

To construct its artificial islands, China builds atop already existing, islands, rocks, and even coral reefs. Building an island that can support airstrips and other military installations requires a lot of sand, however. To gather it, China uses a fleet of dredgers, ships designed to pick up and move materials from the seafloor. These dredgers use large tubes with cutting attachments at the end to grind up material on the seafloor and suck it up. From there, the material is carried through pipes or hoses and dumped on top of reefs, rocks, and other existing formations.

dredgers pouring sand More

CSIS Asian Maritime Transparency Initiative/DigitalGlobe

Once the islands are large and stable enough, China can then lay down cement and build structures on them. The extent of the changes can be striking. For example, below is Fiery Cross Reef in 2006.

fiery cross reef 2006 More