The Cannonball Sea was the last sea to cover North Dakota. This was a shallow, salt-water sea with sharks. North Dakota was hot, swampy, and infested with crocodiles, like the Florida Everglades. The coal beds found in western North Dakota were formed during this period. The coal beds were formed tens to hundreds of millions of years ago by the partial decomposition of land vegetation over time as it was buried. This decomposing plant life, under increasing pressure and temperatures, turned into lignite coal. Learn more