What’s so great about the Great Lakes?

(via TED-Ed)  The North American Great Lakes — Huron, Ontario, Michigan, Erie, and Superior — are so big that they border 8 states and contain 23 quadrillion liters of water. They span forest, grassland, and wetland habitats, supporting a region that’s home to 3,500 species. But how did such a vast and unique geological feature come to be?