Algar do Carvão and Gruta do Natal: visiting Terceira's caves, with Furnas do Enxofre next door

Terceira has two show caves open to visitors, both run by the same caving association, both in the island's interior and a short drive apart. The practical question is when to go, because the caves open only in the afternoon, only on some days outside summer, and one of them has spent most of the last two years behind a building site. This guide covers what each cave is, when they open, what a ticket costs, what to wear, and how to add the fumaroles at Furnas do Enxofre to the same afternoon.

Both caves are managed by Os Montanheiros, a volunteer caving and environmental association. They set the hours, sell the tickets at the entrance and publish changes on their website. There is no online booking for ordinary visits: you turn up during opening hours and pay on the spot, by cash or card. Advance booking is only needed for special openings for groups.

Algar do Carvão is the throat of an old volcano, a vertical shaft you enter from the side and descend by stairs. It is about 12 km north of Angra do Heroísmo, on the central plateau.

The numbers, from the association and from Wikipedia:

• The opening at the top is about 15 by 20 metres.
• From the entrance to the pool at the bottom is about 90 metres of descent.
• You go in through a 44-metre tunnel and then down 338 steps, over several levels.
• The pool at the bottom is fed by rain. It can cover about 400 square metres and reach 15 metres deep, and in a dry summer it can shrink or dry out.
• The walls carry silica stalactites, an unusual formation for a volcanic cave.
• The first recorded descent was on 26 January 1893.

The walls near the top are covered in mosses and ferns, lit by the daylight from the opening. Further down it is darker and wet: humidity is above 90 percent, and water drips from the roof for most of the way. The steps are cemented and slippery in places. The association has closed access to the edge of the lake for safety, so you look at it from the stairs.

A visit takes 20 to 30 minutes. Up to 150 people can be inside at once, so on a busy summer afternoon you may wait at the top.

Algar do Carvão closed in October 2024 so that a new visitor centre could be built next to it. It reopened on 21 March 2026, with the building still unfinished. The association's own notice was blunt: there is no shelter, no rest area, no bar and no toilets, the visit is the ticket and the descent, and the cave may close at short notice, for short or long periods, if the works require it.

At the reopening the construction company gave the end of June 2026 as its target for the visitor centre. Whether it is open by the time you go is exactly the kind of thing to check on the Montanheiros site the day before. In the meantime, use the toilets in Angra or Praia da Vitória before you drive up.

Gruta do Natal is a different kind of cave: a horizontal lava tube, 697 metres long, near the Lagoa do Negro, about 6 km from Algar do Carvão (10 to 15 minutes by car).

• The ceiling is high and the floor slopes gently, so most people walk through it without trouble.
• There are two short flights of stairs, and the rest is the original, uneven cave floor.
• The walls show the marks of different lava flows, small lava stalactites and side ledges.
• The association also runs a more adventurous route through part of the tube.

The name comes from the first Mass celebrated in the cave, on 25 December 1969. It opened to the public as a show cave in 1999. The association kept Gruta do Natal on its normal hours while Algar do Carvão was closed and during the works that followed.

The association's standard calendar is the same for both caves:

• January to March: Tuesdays, Wednesdays, Fridays and Saturdays, 14:30 to 17:00.
• April and May: every day, 14:30 to 17:00.
• June to September: every day, 14:00 to 18:00.
• October to December: Tuesdays, Wednesdays, Fridays and Saturdays, 14:30 to 17:00.

Both close on New Year's Day, Carnival Tuesday, Easter Sunday, 1 December, Christmas Eve, Christmas Day and New Year's Eve.

Algar do Carvão has run on reduced days during the construction work, and the association has warned it can shut without notice. Treat the calendar above as the normal pattern and check the current one before you go, especially outside summer. There are no morning openings at either cave.

Prices as published by Os Montanheiros:

• Algar do Carvão: 10 euros.
• Gruta do Natal: 9 euros.
• Combined ticket for both caves: 15 euros. In March 2026 the Portuguese newspaper Público reported that the combined ticket was not being sold while the visitor centre was unfinished, so do not count on it.
• Children under 12 enter free with a paying adult.

Prices change from year to year; check before you go.

Both caves stay at about 12 to 13 °C inside all year, whatever the weather outside. Montanheiros recommend proper footwear and a waterproof jacket, because water drips from the roof in both caves.

• Wear trainers or walking shoes with grip. Sandals are a poor idea on wet stairs and a rough lava floor.
• Bring a light waterproof layer even on a hot August afternoon.
• The 338 steps in Algar do Carvão come back up too. If stairs are hard for you, Gruta do Natal is the easier of the two.
• Photography is allowed; a tripod needs the association's permission.

Furnas do Enxofre is a field of volcanic vents on the central plateau, a protected natural monument and a short distance from Algar do Carvão. Gas comes out of cracks in the ground at around 95 °C at the surface and about 130 °C half a metre down. It is mostly carbon dioxide, about 95 percent, with around 3 percent hydrogen sulphide, which is the smell. Around the vents the ground is bare, and further out it is covered in mosses and liverworts, about 50 species of them.

A wooden boardwalk loop of about 1 km takes you round the vents in 30 to 40 minutes, and there is no charge. Stay on the boardwalk: the ground next to the vents is hot, and the mosses around them are part of what the site protects. Breathing the gas for long is harmful, so keep moving rather than crouching over a vent.

If you would rather walk than drive between the sites, the official trail PRC10TER (Algar do Carvão - Furnas do Enxofre) is a 6.2 km circuit, graded medium, taking about 2 hours 30 minutes. It starts at the Algar do Carvão car park, follows forestry tracks across the plateau past a disused geothermal borehole, and has an optional 600-metre detour to the fumarole field.

A realistic afternoon from Angra: drive up to arrive when Algar do Carvão opens, go down, drive the 10 to 15 minutes to Gruta do Natal, and stop at Furnas do Enxofre on the way back while the light lasts. In summer, with opening until 18:00, that fits comfortably. In winter, with only two and a half hours of opening, pick the cave that matters more to you and go straight there.

If you are hiking PRC10TER, start in the late morning so you are back at the Algar car park when the cave opens.

Do you need to book Algar do Carvão or Gruta do Natal?

No. Ordinary visits are first come, first served, and tickets are sold at the entrance by Os Montanheiros. Only special openings for groups need booking.

Which cave is better if you only have time for one?

Algar do Carvão, if the stairs are not a problem and it is open: nothing else on the island looks like it. Gruta do Natal is the easier walk and has kept regular hours through the Algar construction work.

How cold is it inside?

Both caves stay at about 12 to 13 °C all year, with dripping water. Bring a waterproof layer and wear shoes with grip.

Is Furnas do Enxofre worth the stop?

Yes, if you are already at the caves. It is free, the boardwalk loop is about 1 km, and it takes 30 to 40 minutes.