Qualibou, Volcanic caldera in Saint Lucia, Caribbean.

Qualibou, also called the Soufrière Volcanic Center, is a caldera 5 km long and 3.5 km wide in southwestern Saint Lucia, partly flooded by the Caribbean Sea. The Pitons, two steep lava domes, stand at its northern and southern edges, and Grand Piton reaches 777 m. Sulphur Springs, an active geothermal area, lies near the caldera's center.

Qualibou caldera formed 32,000 to 39,000 years ago when part of the Choiseul Tuff collapsed into the Caribbean Sea, its debris spreading as a fan off the west coast. Lava domes then rose from the caldera floor; the last eruption inside it came less than 20,000 years ago. The most recent eruption of the Soufrière volcanic centre occurred in 1766 at Sulphur Springs, with phreatic explosions of volcanic explosivity index 1.

The caldera lies in the Soufrière district, and the town of Soufrière takes its name from the volcano. Sulphur Springs, in the caldera's centre, is an active geothermal area where visitors see steaming vents and bubbling mud; the 1766 phreatic eruption there spread volcanic ash over a wide area. The Pitons, two lava domes flanking the caldera, are climbed by visitors who reach the 777 m summit of Gros Piton.

Visitors can reach observation points via marked trails, with local guides available to help explore the geological features. The area requires appropriate protective clothing and caution due to hot surfaces and steam vents.

The caldera's rim reaches 777 metres at Grand Piton, the higher of the two lava domes that predate the caldera itself by 200,000 to 300,000 years. The 1766 phreatic eruption from Sulphur Springs spread volcanic ash over a wide area of the island.