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.

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).

[1] 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.

[1] 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.