This Chinese Dam Is So Big It Altered The Earth's Rotation

Over the course of humanity's long, slow slog through the ages, we've built some incredible structures, and China in particular is not new to grand engineering feats. The Great Wall is roughly 13,166 miles long, constructed over the course of some 2,300 years (from the 3rd century BC to the 17th century AD). While you can't actually see it with the naked eye from space (there are, however, five things you can), its scale is still mightily impressive.

Throughout the millennia, we've stored hundreds of billions of gallons of water behind thousands of dams around the globe. That's had a measurable effect on the position of Earth's poles. And now it seems the Three Gorges Dam hydroelectric project on the Yangtze River in the Hubei province of China is altering Earth's rotation all on its own. 

With construction beginning in 1994 and completing in 2006, the dam contains around 44 billion tons of water, the weight of which has shaved 0.06 microseconds off Earth's rotation, according to NASA scientists. Measuring 607 feet tall, it spans almost 1.5 miles across the Yangtze, and can generate 22,500 megawatts (three times more than the largest dam in the U.S., the Grand Coulee). Approximately 3.67 million people were relocated, and nearly 55 million were impacted because 13 cities and over 1,300 villages were submerged, along with untold archaeological sites and hazardous waste dumps.

The Earth completes one revolution on its axis as it rotates around the Sun every 24 hours, but no two rotations (or two days for that matter) are ever really the same length. So, the 0.06 microseconds the dam is deducting is minimal compared to other natural one-time events. For instance, the 9.1 magnitude earthquake that struck off the coast of Japan in 2011 (causing the Fukushima nuclear accident) actually sped up its rotation by 1.8 millionths of a second. And the Sumatran earthquake in 2004 shifted enough of the Earth's mass to shorten the length of a day by 2.68 microseconds.

The Three Gorges Dam — the largest in the world – isn't going anywhere, so it will continue to alter the planet's geophysics. When water becomes concentrated in one area, Earth's inertia changes, causing a very slight but measurable rotational slowdown. Much like a figure skater moving their arms in and out to control the speed of their spin.

When massive volumes of water are displaced from one spot to another, the Earth's shape actually changes, leading to what's known as "true polar wander," a phenomenon that occurs because the Earth rotates to redistribute mass and restabilize its spin. A report published in Geophysical Research Letters states that all this damned up water has ever so slightly bumped the planet's rotational axis by almost a meter over the last few decades. If the Earth continues to slow down, a shorter day several catastrophic events could occur. Fortunately, we don't need to worry about it for the foreseeable future, but it's something we do need to keep an eye on.