Puerto Rico is a Caribbean archipelago and unincorporated territory of the United States, lying east of Hispaniola and west of the Virgin Islands between the Atlantic Ocean and the Caribbean Sea. It covers about 9,100 km² (3,515 sq mi), and its main island measures roughly 177 km (110 mi) east to west and 65 km (40 mi) north to south. About 60 percent of that island is mountainous. The main landform regions are the Cordillera Central and its two subranges, the Sierra de Luquillo and Sierra de Cayey; the Northern Karst Belt; the narrow northern and southern coastal plains; and the offshore islands of Vieques, Culebra and Mona. The highest point is Cerro de Punta at 1,338 m (4,390 ft), the lowest is sea level along the coast, the longest river is the Río de la Plata at about 74 km (46 mi), and about 120 km (75 mi) offshore the Puerto Rico Trench plunges to about 8,380 m (27,493 ft).
The quickest way to understand the island is to cross it one region at a time, starting in the mountainous centre, then turning to the rainforest in the north-east, the limestone hills and the crowded northern coast, the dry south, and finally the smaller islands.
Cerro de Punta and the Cordillera Central
The Cordillera Central runs the full length of the main island from west to east, splitting it into a wetter north and a drier south. Its rocks are mostly Cretaceous to Eocene volcanic and plutonic formations, ancient island-arc material rather than young volcanoes. Cerro de Punta, the summit of Puerto Rico, is not a volcano either, just the highest knob on the range. It stands on the boundary between the municipalities of Ponce and Jayuya inside the Toro Negro State Forest, reached by a spur off highway PR-143. On a clear day the view from the top takes in most of the island, as far as San Juan more than 120 km (75 mi) away.
East of the main range, the San Cristóbal Canyon on the boundary of Barranquitas and Aibonito marks the start of the Sierra de Cayey, which runs through Aibonito, Cayey, Guayama and Patillas in the south-east. Its summit, Cerro La Santa, reaches 903 m (2,962 ft), and its best-known peak, Cerro Las Tetas, is 840 m (2,759 ft). Altitude makes the interior the coolest part of the archipelago: Aibonito holds Puerto Rico's record low of 3.9 °C (39 °F), while the island as a whole averages about 27 °C (81 °F). Most of the island's more than 50 rivers rise in these mountains, and all 17 of its lakes are reservoirs built on them; none is natural.
El Yunque and the Sierra de Luquillo Rainforest
In the north-east, the small Sierra de Luquillo rises directly behind the coast, less than 40 km (25 mi) east of Old San Juan. Its slopes hold El Yunque National Forest, managed by the U.S. Forest Service since 1903 and the only tropical rainforest in the U.S. National Forest System. It covers 28,516 acres (about 115 km²), mainly in Río Grande, Luquillo and Naguabo, and is the wettest national forest in the U.S. Eight of its peaks top 900 m (2,953 ft). The range's summit is El Toro, at 1,074 m (3,524 ft), but its best-known peak is El Yunque, which gives the forest its name and reaches 1,065 m (3,494 ft). The forest averages about 3,860 mm (152 in) of rain a year and feeds ten rivers, including the Mameyes, Espíritu Santo and Blanco.
Climbing the Luquillo slopes means passing through bands of forest. Tabonuco forest, with trees over 30 m (98 ft), covers the lower slopes to about 610 m (2,000 ft). Above that come palo colorado forest and stands of sierra palm, and on the wettest summits a dwarf cloud forest only about 3 m (10 ft) tall, thick with mosses and bromeliads. Thirteen of the archipelago's 16 species of coquí frog live here, as does the critically endangered Puerto Rican amazon parrot.
The Northern Karst Belt from Arecibo to Camuy
North-west of the Cordillera, younger Oligocene and later limestones overlie the old volcanic core. Rain has dissolved them into the Northern Karst Belt, a landscape of steep, rounded limestone hills, closed sinkholes and underground drainage that holds most of Puerto Rico's caves. Its most famous use was scientific. Engineers built the Arecibo Telescope, a 305 m (1,000 ft) dish, into a natural karst sinkhole near Arecibo. Completed in 1963, it was the world's largest single-aperture telescope for 53 years, until China's FAST overtook it in 2016, and it collapsed on 1 December 2020 after its support cables failed.
Farther west, between Camuy, Hatillo and Lares, the Río Camuy runs underground through a large network of limestone caverns. It is described as the third-largest underground river in the world. More than 10 miles (16 km) of passages, 220 caves and 17 entrances have been mapped, though researchers believe hundreds more caves remain unexplored. Taíno people explored the caves centuries before their modern "discovery" in 1958; Russell and Jeanne Gurnee documented them in a 1973 book. The Camuy River Cave Park built around them has closed twice for storm repairs, after Hurricane Maria in 2017 and Hurricane Fiona in 2022.
San Juan and the Northern Coastal Plain
Between the karst hills and the Atlantic runs a narrow coastal plain, and this is where most Puerto Ricans live. Northern rivers are longer and carry more water than southern ones, because the north catches more rain. The Río de la Plata, at about 74 km (46 mi), is the longest river on the island. The Río Grande de Loíza, with a basin of 751 km² (290 sq mi), carries the largest volume of water.
San Juan, founded in 1521, grew on an islet guarding a deep bay at the north-east end of the plain. The islet now holds Old San Juan and Puerta de Tierra, connected to the main island, and the walls and forts of El Morro and San Cristóbal were listed by UNESCO in 1983. The metropolitan area, spreading across Bayamón, Guaynabo, Cataño and neighbouring municipalities, holds a large share of the territory's roughly 3.2 million people. Farther east along the coast, in Fajardo, Laguna Grande is one of three bioluminescent bays in the archipelago.
Ponce, Guánica and the Lajas Valley in the Dry South
The southern coastal plain, south of the Cordillera Central, receives less rain than the north, so the south is hotter and drier. The territory's record high of 40.6 °C (105 °F) was measured in San Germán in the south-west. Ponce, founded in 1692 and the largest city outside the San Juan metropolitan area, sits on this plain below the mountains, and its municipality reaches north into the mountains as far as Cerro de Punta, which stands on its boundary with Jayuya.
West of Ponce, the Guánica State Forest protects about 10,000 acres (40 km²) of subtropical dry forest across Guánica, Yauco, Guayanilla, Peñuelas and Ponce. It was made a forest reserve in 1919 and a UNESCO Biosphere Reserve in 1981, and it is considered the best-preserved subtropical dry forest in the Caribbean, a sharp contrast with the rainforest of El Yunque on the same island.
West of Guánica Bay lies the Lajas Valley, an east–west trough about 30 km (19 mi) long and 5 km (3 mi) wide between the Cordillera foothills and the Sierra Bermeja. Its fertile soils make it valuable farmland, but it is among the driest places in Puerto Rico and needs irrigation. The low Sierra Bermeja on its southern side holds the oldest rocks in Puerto Rico, about 190 million years old, which may be fragments of Pacific ocean crust.
Vieques, Culebra, Mona and the Puerto Rico Trench
More than 140 smaller islands, islets and cays surround the main island, but only Vieques and Culebra are inhabited year-round. Vieques, about 13 km (8 mi) east of the main island and roughly 32 km (20 mi) long, had 8,249 residents in 2020. The U.S. Navy used much of it as a bombing range until protests forced its departure in 2003, and the former Navy land is now a national wildlife refuge, parts of it still closed for ordnance clean-up. Culebra, about 27 km (17 mi) north-east of the main island, is a small archipelago of its own and, with 1,792 residents, the least populous municipality. Both islands sit on the same shallow Puerto Rico Bank as the Virgin Islands.
To the west, Mona Island lies 66 km (41 mi) offshore in the Mona Passage toward Hispaniola. It measures about 11 by 7 km (7 by 4 mi), has no permanent residents and is managed as a nature reserve for its endemic Mona ground iguana and its seabirds. Beyond all of these, the Puerto Rico Trench marks the boundary of the Caribbean and North American plates. Reaching about 8,380 m (27,493 ft) at the Milwaukee Deep, it is the deepest trench in the Atlantic. The same plate boundary produced the San Fermín earthquake of 11 October 1918, of magnitude 7.1, off the north-west coast near Aguadilla; it set off a tsunami that swept the island's west coast.
For more on how the trench compares with its Pacific counterpart, see our Mariana Trench vs Puerto Rico Trench comparison; for travel across these regions, our Puerto Rico island guide picks up where this tour ends, and our check on whether Jamaica is bigger than Puerto Rico puts its size in Caribbean context.