What region was once covered by the sea?

The Great Plains region of the United States was once covered by the Western Interior Seaway, a shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico. The seaway was formed by the rise in sea level during the Cretaceous Period, about 100 million years ago, and covered an area of about 1 million square miles. The Western Interior Seaway was home to a variety of marine animals, including fish, sharks, and mosasaurs. The seaway began to recede about 70 million years ago, and the Great Plains were formed as the land rose above sea level.