The Gruta do Carvão (Grotto of Coal) is an ancient lava tube located on the outskirts of the municipality of Ponta Delgada (in the civil parish of Santa Clara) on the island of São Miguel in the Portuguese archipelago of the Azores.

Gaspar Frutuoso, the 16th century chronicler and historian wrote: Owing to its scientific, educational and touristic relevance, as well as its natural characteristics, the Gruta do Carvão was classified as a Regional natural Monument by Regional Decree 4/2005/A on 11 May 2005.

[1] The largest lava tunnel on the island of São Miguel, the cave is located in the western edge of Ponta Delgada and extends for 1,650 metres (5,410 ft).

Yet, older documents and field work suggest that the tube actually longer, and may reach a length of 5 kilometres (3.1 mi).

[1] Apart from several cracks, the walls and ceiling are multi-coloured (orange to yellowish) due to the basalt oxidization.