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Sete Cidades ("Seven Cities") is one of the most beautiful natural settings in the Azores, composed of two lakes in the center of a volcanic crater about three miles across. Located on the west side of São Miguel Island, it is the most popular national park in the islands. The direction of the lake as of the photo runs from northeast to southwest and is 5 km in length and about 1 to 2 km in width. This volcano is one of the most active in the Azores in the last 5,000 years.
The two caldera lakes are one of the scenic highlights of the Azores. Since the first eruption about 20,000 years ago, 20 post-caldera eruptions have formed the 500 meters deep caldera walls. At the floor of the caldera six Holocene pyroclastic cones are located. A large group of pleistocene post-caldera trachytic lava domes, lava flows, and pyroclastic-flow deposits is found on the western-to-northern flanks. The most recent eruptions date to the 15th century. These eruptions occurred within the caldera and from submarine vents at the west coast of the island. There have been 4 Strombolian eruptions and 3 Surtseyan eruptions in history within a short distance of the shoreline.

http://en.wikipedia.org/wiki/Sete_Cidades