Terceira

Terceira's 1998–2000 submarine eruption produced rare 'lava balloons' — hollow basaltic spheres up to 1 m in diameter that floated to the ocean surface.

The island contains four distinct volcanic centers — Santa Bárbara, Pico Alto, Guilherme Moniz, and Cinco Picos — each with its own caldera or summit structure.

Angra do Heroísmo, on Terceira's southern coast, is a UNESCO World Heritage Site and was a crucial port for Portuguese maritime exploration and Atlantic trade.

A magnitude 7.2 earthquake struck near Terceira in 1980, killing 61 people and causing severe damage to Angra do Heroísmo's historic center.

Terceira's volcanic system extends well beyond the island's coastline, with the submarine Serreta Ridge representing an active volcanic segment to the west.

The island's name means 'third' in Portuguese, referring to its order of discovery during the colonization of the Azores in the 15th century.

Algar do Carvão, a volcanic chimney and lava tube on Terceira, descends about 90 m into the Earth and features a subterranean lake with rare silica stalactites.

Terceira sits near the Azores Triple Junction where three tectonic plates — North American, Eurasian, and African — interact, making it one of the most tectonically complex volcanic settings in the Atlantic.

The Cinco Picos caldera at Terceira's southeastern end is 7 km in diameter — the largest caldera structure on the island.

Approximately 56,000 people live on Terceira, making it one of the most densely populated volcanic islands in the mid-Atlantic.