Studying the history of a 190-million-year-old chunk of tectonic crust isn’t easy. With subduction constantly tugging these pieces into the mantle, reconstructing their origins is like putting together a jigsaw puzzle without all the pieces—or even the box cover. But this week, scientists say they’ve discovered the unusual origin story of the Pacific tectonic plate, Earth’s largest, Science News reports. Using modern-day magnetic patterns in the Pacific, plus the remnants of old plates in the mantle, researchers put together a surprising history. The way they tell it in their report in Science Advances: The three plates lying beneath the now-extinct Panthalassa Ocean came together in a Y-shaped configuration and began to slide against each other in a pinwheel motion, leaving a triangular gap in the center. Magma filled the gap, the triangle grew, and thus the Pacific plate was born. Because all other modern plates formed by one plate splitting into two, this triple union is all the more radical.