The island of Puerto Rico is a self-governing territory of the United States located on the northern margin of the Caribbean Sea, east of the Dominican Republic and west of the Virgin Islands. It is bordered on the north by the Atlantic Ocean and on the south by the Caribbean Sea. Approximately three times longer east to west than north to south (192 km/119 mi long by 63 km/39 mi wide), it has an area of 9,085 km2 (3,508 mi2). Visited by Christopher Columbus in 1493, during his second voyage, Puerto Rico remained a Spanish colony until 1898, when it was ceded to the United States by Spain as a result of the Spanish-American War. Since 1952 the island has been officially known as the Commonwealth of Puerto Rico. Spanish and English are official languages. At the 2000 U.S. Census its population of 3,808,610 was larger than that of 25 states. San Juan is Puerto Rico's capital and largest city.

Land and Resources. Puerto Rico has three main physiographic regions: the mountainous interior, the coastal lowlands, and the karst area. The mountainous interior is formed by a central mountain chain known as the Cordillera Central in the west and the Sierra de Cayey in the east. The highest peaks, Cerro de Punta (1,338 m/4,390 ft), Cerro Rosa (1,267 m/4,157 ft), and Cerro Jayuya (1,250 m/4,101 ft) are found toward the center of the island in the Cordillera Central near the cities of Jayuya and Orocovis. Another mountain chain is the Sierra de Luquillo in the northeast. The second main physiographic feature is the coastal lowlands, which extend 13 to 19 km (8 to 12 mi) inward in the north and 3 to 13 km (2 to 8 mi) in the south. A series of smaller valleys lie perpendicular near the west and east coasts. The third important physiographic feature is the karst region in the north. This limestone region is an extremely attractive zone of extensive mogotes or haystack hills, sinkholes, caves, limestone cliffs, and other karst features. The karst belt extends from Aguadilla, in the west, to a minor haystack hills formation in Loíza, just east of San Juan.

Soils. Different classification schemes exist for the soils of Puerto Rico. One physiographic approach, based on a scientific classification by the U.S. Soil Conservation Service, can be summarized into five general soil types: humid coastal plains, semiarid coastal plains, humid uplands, semiarid uplands, and humid upland valleys. Another classification by soil scientists at the University of Puerto Rico groups the island's soils into coastal lowlands, alluvium, coastal plains, alluvium in terraces, upland dark, and upland reddish-purple. Traditionally, tropical soils have been looked upon as infertile and unproductive and of poor agricultural value. However, tropical countries provide such high biomass products as sugarcane, bananas, coffee, and tobacco.

Rivers and Lakes. Puerto Rico, due to its relatively short width and its east-west running mountain chain, does not have long rivers or large lakes. Its longest rivers are La Plata, Loíza, and Arecibo, all draining to the north, and the Añasco, draining to the west. There are other perennial rivers, mostly draining to the north and west. Many of the rivers draining south run dry most of the year; nonetheless, with heavy rainfall, they can cause flooding.

Puerto Rico does not have natural lakes, although it has 15 reservoirs, commonly called lakes, formed by damming the main rivers to produce hydroelectric power and water for irrigation. Hydroelectricity accounts for less than 1% of the electricity generated, as most electric power uses petroleum as the energy source. The island has such natural lagoons as the Condado and San Jose in San Juan, Piñones and Torrecillas in Carolina, Joyuda in Cabo Rojo, and Laguna Tortuguero in Manatí-Vega Baja.

Climate. Puerto Rico is a tropical mountainous island lying within the trade-winds belt, which produces relatively constant northeast winds. High temperatures occur throughout the year, averaging 24.4° C (76° F), and there is relatively abundant rainfall. The rugged topography causes varied climatological patterns within this small tropical environment. Mean annual temperature ranges from 21° C (70° F) in the interior mountain chain to 25° C (77° F) in the northern coastal lowlands and 27° C (81° F) in the southern coastal lowlands. The daily temperature range varies from 6 C degrees (11 F degrees) in the northeastern part of the coastal lowlands to 15 C degrees (27 F degrees) in the western parts of the mountainous interior. The range between the mean summer and winter temperatures averages about 3 C degrees (6 F degrees).

Puerto Rico's rainfall pattern is also affected by the topography, with marked differences occurring over relatively short distances. In the El Yunque rain forest, precipitation averages 457 cm (180 in) per year. In contrast, rainfall averages between 76 and 102 cm (30 and 40 in) along the southwestern coastal lowlands, 152 cm (60 in) along the northern coastal lowlands, and 203 to 254 cm (80 to 100 in) along the interior mountains. The rainfall is caused mostly by orographic uplift of the moisture-laden trade winds in the mountains.

Vegetation and Animal Life. Puerto Rico's natural vegetation can be classified into six major zones, according to the Holdridge Ecological Life Zone Classification Scheme. They are the subtropical dry forest, along the south coast; the subtropical moist forest, covering most of the mountainous areas below 305 m (1,000 ft) above sea level and the northern coastal lowlands; the subtropical wet forest, between 305 and 457 m (1000 and 1,500 ft); as well as the subtropical rain forest, the lower montane wet forest, and the lower montane rain forest, all generally more than 305 m (1,000 ft) above sea level.

At the time Columbus arrived on the island, it was almost entirely covered by forest. Most of this original forest vegetation was destroyed as trees were cut for fuel, lumber, agriculture, and grazing. After World War II much agricultural land was abandoned; kerosene, natural gas, and electricity became alternative fuels; and lumber production was reduced considerably. Much of the deforested land reverted to secondary forests or brush, and some reverted to natural pasture.

In an effort to reforest the land and conserve the soil and water resources, some forestland was designated as federal and island forest reserves. Two noted reserves are the Caribbean National Forest, a federal reserve in the Sierra de Luquillo covering 116 km2 (45 mi2), popularly known as El Yunque, and the Guánica Commonwealth Forest Reserve, a xerophytic or dry forest in the semiarid southwestern part of the island. These opposite forest types are less than 161 km (100 mi) apart. The Guánica reserve, about 40 km2 (15 mi2) in size, has been designated a World Biosphere Reserve by UNESCO. About 250 km2 (97 mi2) of forestland have been designated as commonwealth-owned forest reserves. Some of these reserves are coastal mangrove forests.

Puerto Rico has an abundant marine fauna in terms of the variety of species, although not necessarily in terms of quantity. The coastal waters have been overfished. Freshwater fish are also limited. In 1936 the U.S. Department of Agriculture started a program to plant fish in Puerto Rico's rivers and reservoirs, establishing a fish hatchery in Maricao. Tropical freshwater species have been imported from other countries for this purpose. Other species include small amphibians and reptiles, many of which are in danger of extinction. Among the most well known of Puerto Rican amphibians is the coquí, a small tree frog common to the island's humid environments.

Mineral Resources. Although many minerals are found in Puerto Rico, only a few deposits are of sufficient quality and quantity to justify their exploitation. The most noticeable mineral resource is copper, located in a triangular area between Utuado-Lares-Adjuntas in the west central part of the Cordillera Central. Because of the high population density and fragile tropical environment, many island residents have opposed exploiting these deposits. There also has been some petroleum exploration.