Millions of years ago, the Amazon River didn’t flow east to the Atlantic—it flowed west, toward the Pacific. That all changed when the Andes Mountains began to rise, triggered by tectonic activity as the Nazca Plate collided with South America.

As the mountains grew, they blocked the Amazon’s path like a natural dam. With nowhere else to go, the river slowly reversed course, turning eastward over millions of years.

This dramatic shift helped create the Amazon Rainforest, as nutrient-rich sediment from the Andes spread eastward, forming one of the most fertile and biodiverse regions on Earth.