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Pos has several distinctions. It has one of the largest geyser-type craters in the world (1.5 kilometers across) and 300 meters deep. It contains two lakes, one in an extinct crater, one in the fuming main crater. It is in continuing activity, in the form of seeping gases and steam, as well as occasional geysers and the larger eruptions of every few years (the last in 1978). Most practically for the visitor, it is easily reached by a paved road to the peak (2,704 meters - 8,871 feet - above sea level), and the facilities atop the mountain are the best in the national park system.

Get an early start! Aside from the unique experience of ascending a volcano, you’ll want to beat the clouds to the peak in order to get an ocean-to-ocean view. Take a good look at the volcano before you start your ascent. If the cloud cap is dense and widespread, it might be better to wait for another day.

Pos is open to visitors from 8 a.m. to 3:30 p.m. Rain gear will come in handy even in the dry season, when heavy winds whip clouds across the peak. Take a sweater or jacket as well. Temperatures can dip sharply in minutes.

The climate atop Pos is less severe than that on Iraz; the peak is several hundred meters lower, and the steam and gases burn out a smaller area. Vegetation is therefore more abundant. But on a windy day, or when the peak is enshrouded in a dripping pea soup fog, the visitor will find nothing benign about the environment. Nighttime temperatures well below freezing are not uncommon.

Much of the upper part of Pos is cloud forest, the enchanted, cool, moist environment where orchids and bromeliads and vines thrive at every level, along with humble ferns and mosses on the ground. The Pos cloud forest is especially rich in mushrooms and lichens. Parts of the national park are former pastures that are being allowed to return to their natural states; these contain many oak trees. Other sections near the peak are meadow-like, or are characterized by low shrubs and gnarled and twisted trees.

Wildlife in the Pos forest is not abundant, possibly because nearby slopes are farmed intensively. Among the inhabitants are brocket deer, coatis, sloths, cougars, and the Pos squirrel, which has been found only in this vicinity. Birds include several types of hummingbirds, trogons, and the emerald toucanet among more than 70 recorded species.

With increasing volcanic activity in recent years, rains of gases, acids and ash have damaged crops in the surrounding area, and caused authorities to limit access to the park. Currently, visitors may spend no more than 30 minutes at the main crater. They are warned to leave immediately if they feel any irritation of the throat or eyes, not to picnic near the lookout and not to drink from the park’s acidic water system.

The substantial Visitors Center includes an auditorium where a half-hour slide show about the national parks is sometimes given. Orient yourself at the exhibit area before walking around, since you’ll be covering a lot of territory. Aside from a model of the volcano and its craters, there are some wonderful peek-a-boo contraptions where you can try to identify animals by their tracks, samples of volcanic products, volcanic cross-sections, and descriptions of flora.

From visitors’ center, you’ll probably head first to the main crater. Along the walkway you’ll notice the plant called the sombrilla del pobre (poor man’s parasol), which is characteristic of the open areas of Pos. The leaves grow up to two meters across, which explains the name and occasional use of the plant.

Visitors are not allowed to descend into the fuming main crater, but the views from its rim are impressive. At the bottom is a sometime lake formed by rain water, its shade of green changing according to the amount of sulfur it contains at any given time. Water level varies according to the whims and fury and fractures of the earth underneath. Intermittent geyser activity results from water seeping into fissures along the bottom of the lake, then boiling and exploding upward. More likely, you’ll see gas and steam escaping from fumaroles along the lake’s edge. The rim of the crater is burned and strewn with rock and ash, and only a few shrubs struggle for survival in the noxious environment.

After a visit to the active crater, climb to Laguna Botos, the water-filled extinct crater near the highest point on the volcano. The lake is named for an Indian tribe that once inhabited the area.

The last major attraction atop Pos is the nature trail, a run of about half a kilometer through a relatively undisturbed stretch of cloud forest, providing a more interesting route to the crater than the road. The signs in Spanish along the way are more poetic than informative, and some specific labels of trees and plants would be useful (says the gringo). This is the most accessible area of forest of this type in Costa Rica.

Unfortunately, it’s easier to describe many of the features of Pos than actually to see them. The top of the mountain is often clouded over at least partially. However, the clouds shift frequently. If the main crater is obscured at first, take another look before you leave. The shroud might have lifted. The view to either coast, and northward into Nicaragua, might also open up from time to time, so keep an eye peeled.