The mountainous interior of Puerto Rico provides ample opportunities for hill climbing and nature treks. These are especially appealing because panoramas open at the least-expected moments, often revealing spectacular views of the distant sea.

The most popular, most beautiful, and most spectacular trekking spot is El Yunque, the sprawling "jungle" maintained by the U.S. Forest Service and the only rainforest on U.S. soil.

El Yunque is part of the Caribbean National Forest, which lies a 45-minute drive east of San Juan. More than 250 species of trees and some 200 types of ferns have been identified here. Some 60 species of birds inhabit El Yunque, including the increasingly rare Puerto Rican parrot. Such rare birds as the elfin woods warbler, the green mango hummingbird, and the Puerto Rican lizard-cuckoo live here.

Park rangers have clearly marked the trails that are ideal for walking.

A lesser forest, but one that is still intriguing to visit, is the Maricao State Forest, near the coffee town of Maricao. This forest is in western Puerto Rico, east of the town of Mayagüez.

Ponce is the best center for exploring some of the greatest forest reserves in the Caribbean Basin, notably Toro Negro Forest Reserve with its Lake Guineo (the lake at the highest elevation on the island), the Guánica State Forest, ideal for hiking and bird-watching, and the Carite Forest Reserve, a 6,000-acre (2,428-hectare) park known for its dwarf forest.

Equally suitable for hiking are the protected lands (especially the Río Camuy Cave Park) whose topography is characterized as "karst"-that is, limestone riddled with caves, underground rivers, and natural crevasses and fissures. Although these regions pose additional risks and technical problems for trekkers, some people prefer the opportunities they provide for exploring the territory both above and below its surface.

For more information about any of the national forest reserves of Puerto Rico, call the Department of Sports & Recreation at tel. 787/721-2800.

Aventuras Tierra Adentro (tel. 787/766-0470; www.aventuraspr.com) offers the best island adventure tours, focusing on hiking through virgin forests, rock climbing, or cliff jumping. Four different adventures are offered, costing $150 per person, which includes transportation from San Juan. Most of the jaunts take place on weekends.

Safety for Tropical Hikers--When you hike in the tropics, you can quickly become dehydrated and also sustain more serious insect bites and sunburn than you would while hiking in more temperate climes. Take plenty of water and drink it frequently, wear a sun hat, and consider the advisability of long-sleeved shirts and sunscreen to protect yourself from heat exhaustion and sunstroke.