Algar do Carvão is a volcanic vent located on the island of Terceira, in the Azores archipelago of Portugal. The name "Algar" means a natural well or hole in the ground, while "Carvão" refers to the carbon deposits found inside the cave.

The cave was formed by a volcanic eruption around 2,000 years ago, and it is one of the few places in the world where visitors can see the inside of a volcano. The Algar do Carvão cave is approximately 100 meters deep and 15 meters wide, with a lake at the bottom of the cave that is approximately 15 meters deep. The lake is fed by rainwater, and the water is so clear that visitors can see all the way to the bottom.

Inside the cave, visitors can see the various layers of rock and lava that were formed during the volcanic eruption. There are also various stalactites and stalagmites that have formed over time due to water dripping down from the ceiling. The cave is well-lit and has a well-constructed wooden walkway, making it easy for visitors to explore the cave and take in its unique beauty.