China’s Three Gorges Dam, the world’s largest hydroelectric dam, is an absolute beast of an infrastructure project. If you've come across the mindblowing claim that it’s so vast it affects the spin of Earth, the idea is not as ridiculous as it sounds. 

Located in central China’s Hubei province, the Three Gorges Dam spans the longest river in Eurasia, the Yangtze River. It utilizes the flow of water from three nearby gorges – Qutangxia, Wuxia, and Xilingxia  – to spin turbines and generate electricity. 

Claims of its Earth-shifting capabilities seem to have originated in a 2005 post by NASA that explored how the catastrophic Indian Ocean earthquake and tsunami in 2004 affected the rotation of Earth.

It explains how changing the distribution of mass on Earth can have a very small influence on the planet's moment of inertia, a concept in physics that describes how difficult it is to rotate an object around a given axis. It’s the same phenomenon that explains how an ice skater can increase the speed of their spin by tucking their arms close to their body.

Likewise, the spin of Earth can be impacted following an earthquake due to the motion of the tectonic plates. NASA scientists showed that this is exactly what happened in 2004 after the earthquake in the Indian Ocean. By jiggling around the seismic structure of the planet, the earthquake altered its mass distribution and decreased the length of a day by 2.68 microseconds.

In theory, it’s possible for a massive shift of water to do the same. In the 2005 post, Dr Benjamin Fong Chao, a geophysicist at NASA's Goddard Space Flight Center, explains that the giant Chinese dam is able to hold 40 cubic kilometers (10 trillion gallons) of water.

By his calculations, this shift of mass would increase the length of a day by 0.06 microseconds and move the pole position of Earth by about 2 centimeters (0.8 inches). It’s not much, even compared to the marginal effect of giant earthquakes, but it’s fairly significant for a human-made structure.

Humans are influencing the rotation of the Earth in other ways too. A similar effect is currently being caused by climate change and its impact on Earth’s mass distribution. As temperatures increase, the polar ice caps melt and the tropical seas rise, leading to more mass gathering at the planet’s equator than at its poles. In turn, we’re set to experience a slower spin of Earth and very slightly longer days.

The effect is negligible to our everyday perception of life on planet Earth, but it could lead to confusion for super-accurate time-keeping devices like atomic clocks. The ensuing problem has led some scientists to argue that the world will need to account for a negative leap second – ie. a minute with just 59 seconds – within the next decade.