The Rheic Ocean separated two major land masses 430 million years ago. Art by: Christina Ullman, Ullman Design

Call it the ocean that time forgot. About 400 million years ago, the Rheic Ocean played a big role in Earth’s history. When this massive body of water closed, the Appalachians were lifted to Himalayan heights and the planet’s continents slammed together to form the supercontinent of Pangaea. Dinosaurs and early mammals evolved to traverse the large swath of land, spreading life to every corner of the globe.