Poás is an andesitic-basaltic stratovolcano 2,708 m high. It is impressively beautiful and is one of the three volcanoes on the continent that are accessible along a paved road that almost reaches the crater edge.
Subconical in shape, the upper part has three caldera depressions. The main crater is an enormous hollow almost 2 km in diameter and 300 m deep. At the bottom of this crater there is a thermo mineral lagoon some 350 m across. When it occasionally dries out, the sulphur emissions become more intense and produce acid rain that damages the vegetation and the fields on its sides.
To the north of the active crater there is the Von Frantzius cone, the oldest eruption point, which today is currently occupied by a very beautiful cold water lagoon some 400 m across. The volcano experiences plumiforme cyclical eruptions similar to those of a geyser, which consist of columns of muddy water together with steam that rise from a few meters to several kilometres. These eruptions have made Poás famous as the biggest geyser in the world.
The park has four main habitats. One area around the crater is devoid of vegetation or with only a few adapted species like the paddle fern (elaphoglossum lingua). The ara of the arrayans (Pernettya coricea and Vaccinium poasanum) are covered in dwarf vegetation that does not grow over 3 m high.
The stunted forest can be seen along the path between the main crater and Botos lagoon. It is almost impenetrable and is made up of very twisted trees. The cloud forest, very moist and shady, surrounds the Botos Lagoon and the part behind Potrero Grande. Here, most of the trees reach 20 m high and are completely covered in mosses, Hepaticae and other epiphytic plants.
Although animal life in general is scarce, there are lots of birds. Among the 79 species of birds found there, special mention may be made of several humming birds and the (Pharomachrus mocinno), the most beautiful bird on the continent. The Poás squirrel (Sytheosciurus poasensis) is an interesting mammal that is yellowy red in colour with a tail as long as its body.
Poás Volcano is located in the Central Volcanic Cordillera. The administration can be reached via San José - Alajuela - Fraijanes - Poasito (56 km), by paved road. This park has an excellent visitor centre where there are exhibitions, talks and audiovisual presentations. It is equipped with toilets and a café.
The Main Crater and Lagoon paths leave from the offices, and near the Main crater there is a picnic area with tables, toilets and drinking water. Visitors cannot go down into the craters. A bus service operates from San José. In San José and Alajuela there are hotels, restaurants and markets, and along the road you can find restaurants and grocery stores.