Algar do Carvão is Terceira's most dramatic natural sight: a volcanic chimney that plunges 90 metres into the earth, with a descent via lit stairs to a cavern containing stalactites, mineral deposits, and a small underground lake. The cave system was formed by a volcanic eruption roughly 2,000 years ago and is classified as one of the best examples of a volcanic tube in the world.
The descent is straightforward — the path is well-lit and not particularly strenuous — but the interior is genuinely spectacular. Stalactites hang from the lava ceiling, the walls are streaked with mineral deposits in yellows and whites, and the underground lake at the bottom glimmers in the artificial light. The temperature inside stays around 11°C year-round, so bring a layer. Open daily in summer; reduced hours in winter. Admission charged. The nearby Furna do Enxofre (sulphur cave) can be combined with a visit.