Poas has contained a hot, acid crater lake for much of its recent history. The lake water typically has a temperature of 40-50 degrees C, and a pH approaching 1. It is quite shallow, and sometimes disappears to reveal a witches' cauldron of bubbling mud and molten sulphur. Minor eruptions through the lake are quite common, showering the surrounding forest with acid rain. An extensive and extremely complex hydrothermal system is present -- most of the pale colored rocks in the crater wall are extensively altered. Because the lake water buffers the temperature of the system, fumarolic sulphur deposits accumulate, rather than being sublimated as they would in dry conditions (Fig. 6.15, 6.16).

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