The immediate dangers of active volcanoes are well known, but there are hidden dangers as well. Volcanoes in combination with water pose a special kind of lethal effect. This episode includes: an exploding lake in Nyos, West Africa, that sends deadly gases into the valley below, killing 1,7000; volcanic debris that mixes with water to create lahars responsible for countless deaths in the Philippines, and the worst rail disaster in New Zealand history; and volcanologists who believe Mount Rainier may follow its neighbor, Mount St. Helens, in a deadly reign of terror.