Our expedition returned to Volcán Poás today in hopes of reaching hot springs on the mountain's remote west side. We entered the cloud forest at the national park's observation point, peering into the fog for a glimpse of the acid lake far below. The rain-washed path beneath the dense canopy soon led us into the emerald cloud forest most remarkable for the fact that this entire mountainside was devastated in the 1953 eruption—an astonishing example of the process of rapid re-vegetation.

Too soon we left the forest and began the long hike around the crater's rim. After two hours spent traversing a landscape devastated by eruptions, we descended the mountain along a cascading river, and reentered the cloud forest at a lower elevation on the opposite side of the mountain—"the back side," where virgin forest greeted us like an impenetrable green wall. Here we split the party into two groups; one returning to the crater's edge in order to get the photographs you see here today, and the other proceeding into the bush to search for the elusive hot springs. The latter group was led by our local expert, Costa Rican volcanologist Guillermo Alvarado, who was joined by guide Carlos Ramirez, student Alison Shaw, and volcanologist Toby Fischer, who sought water samples from the springs. Toby's report follows:

Today we saw that the majority of the gases are discharging from the 1953 volcanic dome. These fumaroles are much more vigorous than what we were able to sample yesterday. The dome is covered with sulphur, suggesting that at one point the gas temperatures were at least 135°C. We also observed that the gas plume emanating from the dome was particularly extensive—and killing trees in the flanks. This is in drastic contrast to almost no visible plumes at Irazu and Turrialba. These observations suggest that the magma, which is the source of the gases, may be closer to the surface at Poás compared with the other two volcanoes. The gas compositions should test this hypothesis.

We expect a higher HCl/CO₂ ratio at Poás because a higher ratio would indicate a more direct pathway for the gases from the magma to the surface. In contrast, if the ratio is relatively low, then this may show that the gases interact significantly with a hydrothermal system and that the magma is at a deeper level.

Previous geophysical research suggests that a small magma chamber may exist 500 metres below the crater floor of Poás. At Irazu, geophysical data indicates that magma lies two to five kilometers below the surface. So, with our gas studies, we hope to confirm or possibly modify the geophysical interpretations.