The construction of thousands of dams since 1835 has caused Earth’s poles to wobble, new research suggests.

Scientists found that large dams hold so much water they redistribute mass around the globe, shifting the position of Earth’s crust relative to the mantle, the planet’s middle layer.

Earth’s mantle is gooey, and the crust forms a solid shell that can slide around on top of it. Weight on the crust that causes it to shift relative to the mantle also shifts the location of Earth’s poles, the researchers said.