In recent years, China has built up an armada of oceangoing dredging ships, among the most technologically advanced in the world. The country’s annual dredging capacity — the volume of sand and muck it can haul up from underwater — has more than tripled since 2000, to more than one billion cubic meters. That’s more than any other nation. Starting in late 2013, Beijing set a fleet of these dredgers to work raising millions of tons of sand from the sea floor and using it to expand its pieces of territory in the Spratly Islands. Within 18 months, these ships built nearly 3,000 acres of new land.

This type of mega-scale land reclamation is increasingly common. In recent decades, advancing technology has made it easier and cheaper to move ever greater quantities of sand from ever greater depths and deliver it with ever greater accuracy onto predetermined places.

The biggest dredges today are more than 700 feet long; stood on end, they would top a 60-story apartment building. They carry pipes that can pull up sand from 500 feet below the water’s surface. Countries from Singapore to the Netherlands to the United Arab Emirates are using them to expand their coastlines and even build new islands from scratch.