Volcano O -This is one of the largest submarine calderas discovered to date, measuring about 10 km with 500 m of relief. A caldera is a large depression, usually circular or sub-circular, formed by collapse of the summit of a volcano following a large eruption of lava and/or explosive products. The “O” caldera is circular, with several very distinct concentric ring structures within the primary high relief walls. Linear “rift” zones extend SW and N, respectively, from near breaches in its southwest and northern outer walls. Extensive lava flows, defined by high acoustic backscatter zones, extend up to 10 km from the breaches. High- temperature, sulfide-depositing hydrothermal systems occur at and near the intersection of the inner ring faults with the north and south rift zones. Vigorous sulfur-rich diffuse venting occurs at the summit of a young constructional post-caldera cone near the center of the caldera. Our first dive will investigate this intriguing feature and its ecosystems.