Earth’s North and South poles haven’t merely drifted—they’ve apparently taken four significant road trips. A new study in Science finds that over the past 320 million years, Earth’s solid outer shell, aka the crust, has abruptly shifted around its core at least four times, in episodes known as “true polar wander,” reports Smithsonian. Unlike the slow motion of tectonic plates, these bursts of movement can change climate systems, per New Scientist.
Researchers spotted the events—the most recent of which took place 20 million to 30 million years ago—by modeling how sea levels would rise and fall if the planet’s bulk moved beneath its equatorial bulge of water, per Smithsonian. The scientists say that these episodes can disrupt ocean currents, climate, and ecosystems, noting that they’re happening today at about 4 inches a year, largely driven by melting ice. What sets them off remains unknown.