The Great Lakes were not once a "sea" in the sense of being an open, saltwater ocean, but their basins were covered by ancient inland seas hundreds of millions of years ago. During the Paleozoic Era, much of central North America was submerged under shallow, tropical saltwater seas, which deposited the thick layers of limestone, sandstone, and salt that now form the bedrock of the Great Lakes region. However, the lakes themselves as we know them today are relatively young and are entirely freshwater. They were formed roughly 10,000 to 14,000 years ago at the end of the last Ice Age. As the massive Laurentide Ice Sheet, which was over two miles thick, retreated toward the north, it carved out deep basins in the Earth's crust and filled them with melting glacial water. This process created the largest system of fresh surface water on the planet. While you can still find fossilized saltwater coral and ancient marine life in the rocks surrounding the lakes—remnants of that much older Paleozoic sea—the modern Great Lakes are a product of glacial activity and have remained freshwater since their inception following the retreat of the glaciers.