La Soufrière Sulphur Springs were pretty neat. This picture shows a series of mud pots that were boiling. The sulphur springs are inside the Qualibu caldera, an collapse feature that formed between 40,000 and 20,000 years ago. The University of the West Indies has the best website on St. Lucian geology that I could find. They talk specifically about the Soufrière Volcanic Centre and the Sulphur Springs.