Gulf of Mexico numerous bays and smaller inlets. A number of rivers empty into the gulf, most notably the Mississippi River and Rio Grande in the northern gulf, and the Grijalva and Usumacinta rivers in the southern gulf. The land that forms the gulf's coast, including many long, narrow barrier islands, is almost uniformly low-lying and is characterized by marshes and swamps as well as stretches of sandy beach. The Gulf of Mexico is an excellent example of a passive margin. The continental shelf is quite wide at most points along the coast, most notably at the Florida and Yucatán Peninsulas. The shelf
Gulf of Mexico shallow continental shelf waters. The basin contains a volume of roughly 2,500 quadrillion liters (550 quadrillion Imperial gallons, 660 quadrillion US gallons, 2.5 million km or 600,000 cu mi). The Gulf of Mexico is one of the most important offshore petroleum production regions in the world, comprising one-sixth of the United States' total production. The International Hydrographic Organization defines the southeast limit of the Gulf of Mexico as follows: A line leaving Cape Catoche Light () with the Light on Cape San Antonio in Cuba, through this island to the meridian of 83°W and to the Northward along this meridian