This is one of the wettest areas in the country. As much as 5,500 mm falls on the highest hills. The vegetation, one of the richest and most diverse in Costa Rica, is botanically very similar to South America. The main habitats are mountain forest that covers over half the park and contains the greatest variety of species of fauna and flora in the area; the cloud forest that occupies the highest parts is very rich in oaks (Quercus insignis) and (Quercus rapurahuensis), and in tree-ferns; high plains forest, occupying the alluvial part of the park; swamp forest that is flooded almost all year ; holillo forest with the holillo palm predominant (Raphia taedigera); herbaceous freshwater swamp and Corcovado lagoon over 1,000 ha in area, covered in grasses and bushes and representing an exceptional refuge for animals and birds; mangrove swamp in the lagoons of rivers Llorona, Corcovado and Sirena, and coastal, vegetation.
There are 500 species of trees in the whole park, representing a fourth of all the tree species in Costa Rica. Some, like the endemic are rare poponjoche (Huberodendron alleni), the purple heart (Peltogyne purpurea), the silk cotton tree (Ceiba pentandra) and the espave (Anacardium excelsum), reach or exceed 50m high. In the mountains, two species of wild cocoa (Theobroma angustifolium) and (Theobroma simiarum) occur.
The fauna of Corcovado is as rich and varied as its plants. 140 species of mammals, 367 birds, 117 amphibians and reptiles and 40 freshwater fishes are known to occur there, and there are estimated to be 6,000 insects,. The park holds the biggest population of scarlet macaw (Ara macao) in the country. Some of the threatened species found there are: Baird's tapir (Tapirus bairdii), giant anteater (Myrmecophaga tridactyla) and five of the six species of cats found in Costa Rica; namely puma (Puma concolor), ocelot (Leopardus pardalis), jaguaroundi (Herpailurus yaguarondi), margay (Leopardus wiedii) and jaguar (Panthera onca).
On wide Llorona Beach, four species of marine turtles lay their eggs in relatively large numbers. In the sea area off Corcovado, dolphins, bull sharks (carchahinus leucas) und three species of whales, including the humpback whale (Megaptera novaeangliae), can often be seen.
Given its extraordinary biological diversity, Corcovado is currently an important centre for research into moist tropical forest; in Sirena there is a biological station with facilities to carry out research. This park and the rest of the peninsula was an important centre of settlement of Pre-Hispanic peoples; archaeological sites have been found practically on the edged of all paths within the park.
Corcovado is in the southwest of the Osa Peninsula. The offices are in Puerto Jiménez. Sirena has a landing strip and several paths lead off from there, the most important ones being Río Claro , San Pedrillo (along the beach and through the forest), Ollas, Río Sirena, Los Espaveles and Río Pavo. The El Mirador and Leopna to Sirena parths are in Leona. El Mirador and Patos to Sirena are in Los Patos, and the La Catarata, Río Pargo and San Pedrillo to Llorona are in San Pedrillo. There are camping and picnic sites in Sirena, La Leona, Los Patos and San Pedrillo, with tables toilets and drinking water.
Access to Sirena is by light plane from San José. By land, it is possible to get from Puerto Jiménez to La Leona (44km). A collective bus service operates between Puerto Jiménez and La Leona. In Puerto Jiménez there are hotels, boarding houses, restaurants and markets. Private nature reserves with cabins and biological stations have been set near the park.