About 75% of Puerto Rico's land area consists of hills or mountains too steep for intensive commercial cultivation. The Cordillera Central range, separating the northern coast from the semiarid south, has the island's highest peak, Cerro de Punta (4,389 feet–1,338 meters). Puerto Rico's best-known peak, El Yunque (3,496 feet–1,066 meters), stands to the east, in the Luquillo Mountains (Sierra de Luquillo). The north coast consists of a level strip about 100 mi (160 km) long and 5 mi (8 km) wide. Principal valleys are located along the east coast, from Fajardo to Cape Mala Pascua, and around Caguas, in the east-central region. Off the eastern shore are two small islands: Vieques, with an area of 51 sq mi (132 sq km), and Culebra, covering 24 sq mi (62 sq km). Uninhabited Mona Island (19 sq mi–49 sq km), off the southwest coast, is a breeding ground for wildlife.
Puerto Rico has 50 waterways large enough to be classified as rivers, but none is navigable by large vessels. The longest river is the Rio de la Plata, extending 46 mi (74 km) from Cayey to Dorado, where it empties into the Atlantic. There are few natural lakes but numerous artificial ones, of which Dos Bocas, south of Arecibo, is one of the most beautiful. Phosphorescent Bay, whose luminescent organisms glow in the night, is a tourist attraction on the south coast.
Like many other Caribbean islands, Puerto Rico is the crest of an extinct submarine volcano. About 45 mi (72 km) north of the island lies the Puerto Rico Trench, at over 28,000 feet (8,500 meters) one of the world's deepest chasms.