Costa Rica’s southwesternmost region is a distinct oblong landmass, framed on its east side by the Fila Costeña mountain chain and indented in the center by a vast gulf called Golfo Dulce . Curling around the gulf to the north is the mountainous hook-shaped Peninsula de Osa  and, to the south, the pendulous Peninsula de Burica .
North and south of the gulf are two broad fertile plains smothered by banana plantations—the Valle de Diquis , to the northwest, separating the region from the Central Pacific  by a large mangrove ecosystem fed by the Río Grande de Térraba, and the Valle de Coto Colorado, extending south to the border with Panamá .
Nature lovers with a taste for the remote and rugged will be in their element. Star billing goes to the Osa Peninsula, smothered in a vast tract of pristine rainforest filled with the stentorian roar of howler monkeys, the screeches of scarlet macaws, and the constant dripping of water. Much of the jungle—a repository for some of the nation’s greatest wildlife treasures—is protected within a series of contiguous parks and reserves served by remote jungle lodges.
The region is the largest gold source in the country, as it has been since pre-Columbian times. In the early 1980s, gold fever destroyed thousands of hectares of the Osa forests: The physical devastation was a deciding factor in the creation of Corcovado National Park . Rivers such as the Tigre and Claro still produce sizeable nuggets; former gold miners have turned to ecotourism and today lead visitors on gold-mining forays.
The waters of the Golfo Dulce are rich in game fish, and the area is popular for sportfishing. Whales occasionally call in, and three species of dolphin—bottle-nosed, black spotted, and spinner—frolic in the gulf, which is charged by luminescent microbes after sunset. Though the gulf is protected and relatively calm, surfers flock for the waves that wash the southeast tip of the Osa Peninsula  and push onto the beaches of the Burica Peninsula , where indigenous communities exist in isolation within the mountains.
Offshore, west of Corcovado, is craggy, desolate Caño Island , and way, way out to the southwest is the even more desolate Cocos Island , where surrounding waters are a venue for some of the world’s finest diving.
This is one of Costa Rica’s wettest regions. Be prepared for rain and a lingering wet season: the area receives 4–8 meters of rain annually! Violent thunderstorms move in October–December.