The day after Christmas in 2004, a tsunami swept over Indonesia, Thailand, and 12 other countries. It killed more than 230,000 people in the most lethal natural disaster in recorded history.

Most of us remember the news footage from those grim days. What you may not remember is that the tsunami was caused by an earthquake. Scientists now know it was the most powerful earthquake in 40 years, a 9.3 on the Richter scale. Its epicenter was below the ocean floor, about 100 miles west of Sumatra.

The quake had the longest duration ever recorded, about 10 minutes of continuous motion as Earth’s crust ripped to form a 50-foot cliff on the seafloor. The tear continued moving north for almost an hour, finally extending more than 750 miles.

It was the huge volume of water displaced by this movement that caused the tsunami.

The quake shook the ground everywhere on Earth and triggered powerful aftershocks and earthquakes as far away as Alaska. The entire planet vibrated for weeks.

GPS data showed changes in the surface of most of the Eastern Hemisphere. Masses inside Earth shifted, which moved the location of the North Pole by an inch. Even the shape of the globe changed—very slightly, but enough to increase its rotational speed and shorten the length of the day by 3 microseconds.

December 26, 2004, was a shocking day in Earth’s history, for humans and the planet itself.