A survey of the Brothers caldera volcano (Kermadec arc) with the autonomous underwater vehicle ABE has revealed new details of the morphology and structure of this submarine frontal arc caldera and the geologic setting of its hydrothermal activity. Brothers volcano has formed between major SW-NE–trending faults within the extensional field of the Havre Trough. Brothers may be unique among known submarine calderas in that it has four active hydrothermal systems, two high-temperature sulfide-depositing sites associated with faulting on the northwestern and western walls (i.e., the NW caldera and W caldera hydrothermal sites, respectively), and gas-rich sites on the summits of the constructional cones that fill most of the southern part of the caldera (i.e., the Upper and Lower cone sites). The 3.0- × 3.4-km caldera is well defined by a topographic rim encompassing ∼320° of its circumference and which lies between the bounds of two outer half-graben–shaped faults in the northwest and southeast sectors. There is not a morphologically well defined continuous ring fault (at the map resolution), although near-vertical scarps are present discontinuously at the base of sections of the wall. The width of the wall varies from <200 m at its southwest portion to ∼750 m on its northern section. The widest part of the wall is its northwest sector, which also has the largest documented area of hydrothermal alteration and where sea-floor magnetization is lowest. In addition to primary northwest-southeast elongation and southwest-northeast structures caused by faulting within the regional back-arc strain field, there are also less well developed west-southwest–north-northeast regional structures intersecting the volcano that is apparent on the ABE bathymetry and at outcrop scale from submersible observations. Asymmetrical trap-door–style caldera collapse is considered a possible mechanism for the formation of the Brothers caldera.