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Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

Creating the man-made Palm Islands in Dubai took planning and precise engineering. The Palm Jumeirah was the first of the Palm Islands to be built, and started with a crescent-shaped breakwater to protect the reclamation work from rough seas. At its deepest, the breakwater stands in 35 feet (11 meters) of the ocean and rises to 13 feet (4 meters) above sea level at low tide and is designed to protect The Palm against the worst sea conditions imaginable. Engineers placed openings on either side of the crescent to allow the seawater to be refreshed every 14 days in order to prevent the water trapped within the island fronds from becoming brackish.

The bottom of the crescent is a small hill of sand, taken from the sea and not the desert because, according to the engineers, the sea sand is more environmentally sustainable and more seismically stable. The engineers used a Differential Global Positioning System to check the accuracy of the sand placement to within one centimeter. A sheet of water-permeable geotextile over the sand ensures it remains in place and is not washed away.

On top of the sand lies a protective layer of "small" rocks, each weighing up to a ton. On the inside of the crescent lies a "toe" of rocks weighing between one and four tons which were lifted into place by a floating crane and then pushed and shaped with underwater excavators. Above the layer of smaller rocks are two more layers of "armor," made up of rocks weighing as much as six tons.

With the crescent securely in place, 173 million cubic feet (5 million cubic meters) of rock were positioned to create the foundation of the inner island. Dredgers then began working around the clock to transfer 3.25 billion cubic feet (91 million cubic meters) of sand to build up the landmass. Once the dredging was complete, it was vital to settle the sand before it was built on—a natural process that normally takes millions of years. So the sand underwent a process called vibro-compaction, which should mean that any further settlement should be less than one inch over the next 50 years.

—Marisa Larson