The Algar do Carvão (Cavern of Coal), Terceira Island (Azores), Portugal

The Algar do Carvão (Cavern of Coal) is an ancient lava tube or volcanic vent located in the central part of the island of Terceira in the Portuguese archipelago of the Azores. It lies within the civil parish of Porto Judeu in the municipality of Angra do Heroísmo. 

Their are few very few places in the world allow you to explore and go inside a volcano. One of these places is the Algar do Carvão, located in the Azorean island of Terceira. The name Algar do Carvão means “coal pit” because the cave walls are formed from dark black lava.  To enter the cave you need to go down the volcano chimney that is lava free today, uncommon in most volcanoes, and a tunnel and staircases offer access to the caves below. At the bottom there is a crystal clear lake formed by rainwater, with unique stalactites and stalagmites.

The Algar do Carvão is, in fact, one of the few volcanoes in the world you can visit and the only one in which visitors can explore a volcanic chimney and secondary magma chambers. The volcanic eruption that took place at Algar do Carvão 3,200 years ago originated into volcanic rock highly rich in silica. Then, a second eruption, 2,000 years ago happened in the same place but with basaltic lava that formed several magmatic chambers.

Thanks to this fluid lava the chimney and the magma chambers were left empty when it all settled down. Algar do Carvão is now the only place on earth where you can see these two structures, the chimney and chambers without being cooked alive. What stands out as unique in the world is the stalactites and stalagmites formed by amorphous silica – Alvar do Carvão actually has the biggest concentration in one place.

Tours and the general maintenance of the volcano cave is in charge of local association “Os Montanheiros.” The first group of Montanheiros went to the volcano interior Montanheiros in 1963 and in 1968 they led the first visitors. Visits take place all summer and have to be scheduled in advance in the winter. They also organize visits to the Gruta do Natal, on Terceira.

A crystal-clear freshwater lake covers the base of Algar do Carvao, formed by the constant rain. Nuno estimates it’s about 39ft (12m) deep, although it can rise to 82ft (25m). During periods of drought, the water completely dries up. Down here, the dripping has an eerie echo. The only lifeforms this far down are bacteria, and a fleet of endemic spiders, centipedes and beetles found in the Azores that lurk in the cracks and crevices.

Before climbing back up the 338 steps, I take a moment to let it all sink in. This is where molten magma exploded out from the depths of the Earth 2,000 years ago. The same magma that created this tiny island in the middle of the Atlantic Ocean.

The existence of the cavern had been known for a long time, but the depth and lack of ambient light, made any descent difficult, owing to the vertical descent, and delayed any real exploration.

On 26 January 1893, the first descent, "...with the use of a simple rope, was accomplished by Cândido Corvelo and Luis Sequeira."

The second descent was made in 1934, by Didier Couto, who produced the first rough map of the interior. This drawing, based on visual observations alone (rather than measurements), turned out to be quite accurate.

On 18 August 1963, a group of enthusiasts organized a descent using a "chair" platform suspended on a nylon line and, later, a harness.

With the advent of portable lighting systems, it was possible to re-examine the observations first-hand, including the most remote and narrow sections of the cavern. The site was open to the public in the late 20th century, through weekly organized tours (except weekends and holidays) through the summer (from May to September). Access to the site is also possible through the Montanheiros (Mountaineers) organization, the group responsible for matters associated with the Algar.

The word, algar, is a Portuguese word that denotes a natural cavity in the earth that, unlike most caves or caverns, is more vertical in its orientation, like a well. The Portuguese word derives from the Arabic word, al-Gar, which means "the crease", while Carvão means "coal" in Portuguese. The Algar do Carvão is not a source of coal, and the term carvão has also been used in the Portuguese vernacular akin to "blackened", "sooty" or "burnt", referring to the dark composition of the rocks.

Terceira is the site of four large volcanoes (Pico Alto, Santa Bárbara, Guilherme Moniz, and Cinco Picos) grouped along a basaltic fissure zone that transects the island from northwest to southeast. The Algar do Carvão is directly associated with the caldera of the Guilherme Moniz volcano, but it is part of the same complex as Santa Barbara to the west, Pico Alto to the north, and Guilherme Moniz, which is a short distance to the south of the Algar do Carvão site. Algar do Carvão is situated at 583 metres (1,913 ft) above sea level.

The 40.5 hectares (4,360,000 sq ft) extent was classified as a Regional National Monument (Portuguese: Monumento Natural Regional) due to its peculiar volcanological characteristics, in addition to its environmental ecosystem. The mouth of the Algar consists of a 45 metres (148 ft) vertical passage to the interior, that reaches a ramp of debris and gravel. From here there is another decline to the clear waters of the interior pool, approximately 90 metres (300 ft) from the level surface. The pool is supported by rainwater, and can reach as deep as 15 metres (49 ft) depth or become dry in summer months, owing to little or no precipitation.

The cave itself is remarkable for the unique mineralogical characteristics of its silicate stalactites. The Algar is populated by a rich plant tapestry, that covers the mouth of the cone structure, including various endemic species. Further, there are several invertebrate species that make the cavern their homes, such as the cavernous spider Turinyphia cavernicola and other species of Troglofauna, like the centipede Lithobius obscurus azorae, the springtail Pseudosinella ashmoleorum and ground beetle Trechus terceiranus. Finally, there is a distinct presence of various moss, including those on the Red Data Book of European Bryophytes (ECCB), like the Alophosia azorica and Calypogeia azorica, among others