Gruta do Natal: the Lava Tube Where Mass Has Been Held Since 1969

Not every volcanic cave in the Azores has a clear origin. Gruta do Natal is one of these geological mysteries — but what it lacks in scientific certainty, it makes up for in human history.

An origin still unexplained

Located near Picos Gordos, within the Santa Bárbara and Mistérios Negros Forest Nature Reserve, in the parish of Altares, Gruta do Natal is a basaltic lava tube 697 metres long, 12 metres high, and up to 7 metres wide. To this day, researchers have no conclusive evidence of its exact origin, believed to result from lava flows from fissure eruptions that occurred thousands of years ago.

Three names for one cave

Discovered in 1838, the cave was first called Galeria Negra (Black Gallery). Later, when early explorations found the remains of a horse’s skeleton inside, it became known as Gruta do Cavalo (Horse Cave). The current name only emerged after the local group “Os Montanheiros” held, on 25 December 1969, the first Christmas mass inside the cave — a tradition kept up whenever possible ever since.

One of Terceira’s 16 caves

Terceira Island has 16 known lava tubes, part of a total of 250 across the entire Azorean archipelago. Gruta do Natal, roughly 12,000 years old, is only partially open to visitors, but remains one of the island’s two best-known underground attractions, alongside Algar do Carvão.

A visitor centre built in 1998

It was only in 1998 that Os Montanheiros managed to move forward with the cave’s tourist development, building a support house that houses the generator, protects the cave from vandalism, and displays a photo exhibition of baptisms, masses, and weddings held there over the years.

Planning your visit

• Location: Altares parish, Angra do Heroísmo municipality.
• Access: internal route with two distinct paths, in and out, via stairs and preserved original flooring.
• Guided tours: available year-round, with a knowledgeable team on local geology.

Best time to visit

Any time of year works for a visit, since the interior temperature stays stable.

Nearby highlights

Algar do Carvão, the island’s other great volcanic cave, is a short drive away.

After Gruta do Natal, head to Algar do Carvão to compare the two major volcanic cavities that make Terceira a landmark for Azorean speleology.