Furna do Enxofre

Inside the caldera at the southern end of Graciosa - itself a UNESCO biosphere reserve - a lava dome collapsed to leave a cavern about a hundred metres across and eighty high, and at its bottom sits a cold lake of sulphurous water with a mud pool and fumaroles alongside. Access is through a tunnel driven into the rock and then a spiral staircase of 183 steps built inside a stone tower, descending into the dark; a natural opening in the roof lets a shaft of light in around the middle of the day, which is when the cavern is at its most striking. Carbon dioxide accumulates at the floor and is monitored, and visits are guided with entry restricted at times when levels are high. Graciosa is the smallest and least visited of the central Azores islands, which makes reaching it the main obstacle.

What you'll find

• The cavern - about a hundred metres across and eighty high inside a collapsed lava dome.
• The sulphur lake - cold water with a mud pool and fumaroles at the bottom.
• The spiral stair - 183 steps down inside a stone tower.
• The light shaft - sun entering through the roof opening around midday.

When to go

Year-round; midday for the light through the roof opening. One to two hours. Guided entry at set times, so check the schedule.

Things to know

Carbon dioxide levels are monitored and the cavern closes when they rise - this happens and cannot be argued with. 183 steps down and back up. Cold, damp and strongly sulphurous. Graciosa has limited flights and ferries; getting there is the hard part.

Nearby

Santa Cruz da Graciosa is north; the Carapacho thermal baths are on the southern coast.

Key facts