By Warren Cole Smith, The Stream - The North American Inland Sea, sometimes called the Western Interior Seaway, once covered a huge portion of North America. It was 2500 feet deep in places, and 600 miles wide. Louisiana and Oklahoma were underwater. So was Texas, except for a few islands we now call the Texas Hill Country. Much of Canada, too. Most of the states of the Intermountain West — Colorado, New Mexico, Utah, Wyoming and others — were a vast mountainous island.