Central America, North America southeast of Mexico, land bridge to South America, separating the Pacific Ocean from the Caribbean Sea. Its north and south land boundaries, about 1,100 mi (1,770 km) apart, are the Isthmus of Tehuantepec in Mexico and the valley of the Atrato in Colombia. Within this area are the republics of Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, Belize, and Panama. Most of the people are of Spanish or Native American ancestry, living as farmers in the mountain valleys or working in the forests or mines. The dominant feature of the region is a string of mountain ranges characterized by great volcanic activity and earthquakes. They are part of the same mountain system as the islands of the Greater and Lesser Antilles. Most of the mountains are no more than 6,000 ft (1,829 m), though some volcanoes are higher than 10,000 ft (3,048 m), with Mt. Tajumulco in Guatemala rising to 13,845 ft (4,220 m). The volcanic deposits in the northwest have produced very fertile highlands. Relatively level tracts of land occur on the Yucatán Peninsula and along the coast. These limestone plains of the peninsula are the most extensive in Central America, and support dense tropical forests. The rolling coastal lowlands appear insignificant by comparison, broadening to a width of only 90 mi (145 km) along the Mosquito Coast (on the Caribbean) and never exceeding 30 mi (48 km) along the Pacific coast. A trough, the Nicaraguan Depression, divides the northern part of the isthmus from the narrower southern part and encloses lakes Nicaragua (3,000 sq mi/7,770 sq km) and Managua (386 sq mi/1,000 sq km), the largest in Central America. The climate is basically tropical, but is drier on the Pacific side. The considerable variations in temperature and rainfall are due mainly to differences in elevation. The 2 early civilizations of the region were the Mayan and the Toltec. The Maya reached their highest cultural state between A.D. 300 and 900. They were familiar with mathematics, astronomy, and agricultural techniques; they built cities of stone, erected fine monuments, and developed intricate handicrafts. The Toltec of central and southern Mexico spread their influence to the Yucatán Peninsula and into Guatemala. The Spanish conquerors arrived in the early 1500s to destroy what were already dying civilizations. Hernando Cortes completed the conquest of the whole area by 1525, and it remained a Spanish colony for nearly 300 years. Foreign rule ended in 1821, when a sympathetic Spanish governor was elected the first head of the independent United Provinces of Central America. The union was dissolved in 1838, and the 5 states (Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua) became independent. Subsequent attempts to reestablish the confederation have failed. A looser form of cooperation was begun with the creation of the Organization of Central American States in the 1950s. Costa Rica is politically the most stable country; the others have suffered from external conflicts, dictatorships, and revolutions.