Algar do Carvão

Algar do Carvão is one of the rare places on Earth where you can walk down inside the throat of a volcano. In the middle of the island of Terceira, among pastures and cedar scrub, a dark opening about 15 by 20 metres leads into a vertical chimney that drops some 90 metres to a pool of rainwater. Most volcanic vents seal themselves with collapsed rock or solidified lava; this one was left open when the magma drained back down after an eruption, leaving a hollow shaft and a pair of vaulted chambers that can be visited.

The descent begins through a tunnel cut in the 1960s and continues on a long stairway — more than three hundred steps — that winds down past walls of basalt, black obsidian and reddish iron oxides. Looking back up, the mouth of the chimney is framed by ferns and mosses hanging into the light, an image that has become one of the best known of the Azores. Lower down, the vaults are hung with milky-white stalactites and stalagmite-like crusts made not of limestone but of amorphous silica, deposited by water seeping through the volcanic rock. Silica formations on this scale are very unusual in caves anywhere, and they are the feature specialists come to see. At the bottom lies a clear lagoon of roughly 400 square metres, fed by rain and small springs; after a wet winter it can be about 15 metres deep, and in dry years it almost disappears.

The cave is the product of an eruption dated to a little over two thousand years ago, linked to the Pico do Carvão cone on the edge of the old Guilherme Moniz caldera. Its existence was long known, but the first recorded descent was only made on 26 January 1893, by two men on a simple rope; a rough interior profile followed in 1934, and in 1963 a group of local enthusiasts began organised descents. That group became Os Montanheiros, the caving and mountaineering association that still looks after the site and guides visitors. The area around the cave is protected as a regional natural monument, and the shaft also shelters cave-adapted invertebrates and rare endemic mosses.

Getting there

The cave lies in the centre of Terceira, about 12 km by road north of Angra do Heroísmo, the island's historic capital. The nearest airport is Lajes Airport (TER), about 11 km away. There is no regular public transport to the site; most visitors come by car or taxi.