Poás volcano is a powerful symbol of the geothermal forces that formed Costa Rica. When the mist and clouds part you'll see the sulfuric, bubbling, green rain fed lake at the bottom, surrounded by smoke and steam rising from fumaroles. Water from the lake is constantly seeping through cracks in the hot rock, evaporating and building pockets of steam. When the pressure in these pockets exceeds the weight of the water above, the steam breaks through in geysers that rocket up to 820 feet (250 meters) high. Don't worry about getting a shower though, the crater is 1,050 feet (320 meters) deep. At almost a mile (1.6 km) across it's also the largest active crater in the world.

Poás is active, but don't expect to see a full fledged eruption or even any lava flow here, the most recent period of eruptive activity ended in 1954. The last major activity was in 1910 when nearly a million tons of ash was ejected along with an immense column of smoke and steam.

Early morning before the clouds roll in is the best time to visit Volcán Poás. (click for more photos of Poás)

A modern visitors center (wheelchair accessible) and small museum explains and interprets the geothermal and ecological attractions in the park. A cafe serves coffee and hot drinks—as much to wrap your hands around to warm them as anything else—sandwiches, and snacks.

The volcano provides an excellent if extreme example of the effects of acid rain. Around the caldera, and for several miles downwind, the vegetation is stunted brown and black by the tainted moisture that precipitates from the omnipresent clouds near the peak.