Located in the western part of Ponta Delgada city, Gruta do Carvão is the longest lava tunnel of São Miguel island and it extends for 1912 metres divided in three separate sections, one to the North ("Paim" Section) with a length of 880,2 metres, one to the South ("João do Rego" Section) with an extension of 300 metres and an intermediate one ("Secadores de Tabaco","Rua de Lisboa" Section) that is 701,8 metres long. However some old documents and field observations indicate a bigger length than the mapped nowadays and that it might have been more than 5 km long, since coastline until the proximities of Serra Gorda, in Arrifes.
Gruta do Carvão is located in a region of São Miguel geologically known as the "Complexo Vulcânico dos Picos", an area of fissure volcanism composed of over 250 scoria cones and lava flows of basaltic nature that roughly covers the area between Capelas and Ribeira Grande (North Coast) and between Ponta Delgada and Lagoa (South Coast).The cave develops itself in a long basaltic flow oriented N-S that reached the sea in the west part of the city of Ponta Delgada. It´s main eruptive center is located in the axial zone of the "Complexo Vulcânico dos Picos".
The cave´s age is comprehended in the gap between 5000 to 12000 years old, this according to C14 dating method and by analyzing the pyroclastic materials deposits (ash and lapilli), possibly emitted from the Sete Cidades volcano and/or the Fogo volcano, that cover this cave. It´s assigned to Gruta do Carvão an Holocene age.
Due to its location, size, variety of geological structures and volcanic phenomenons that are associated with, Gruta do Carvão was classified as a Regional Natural Monument by the Regional Decree Nº4/2005/A of May 11th.