Cavi Angusti represent a series of large irregular depressions localized in part of the south circumpolar area previously mapped as the Hesperian-aged Dorsa Argentea Formation. Their origin has primarily been interpreted to be due to eolian deflation or subglacial melting. We use MGS MOLA and MOC data to analyze the largest of these features (~100 ¿ 50 km, and up to about 1500 m deep). These data reveal terraced interiors, centrally located equidimensional and elongated edifices, and lava-flow-like structures that strongly suggest that this basin formed as a result of magmatic intrusion and extrusion, causing heating and melting of a volatile-rich substrate and drainage and loss of the liquid water. Volume estimates and heat transfer calculations are consistent with a mechanism involving a combination of intrusion and extrusion very similar to that observed to be responsible for Icelandic subglacial eruptions and meltwater generation. Mounds and ridges in the floors of other depressions suggest that this mechanism may have operated in at least several other features of the Cavi. Eolian activity, sublimation, and solution are also likely to have played a role in further modification of these features. Meltwater from basin formation appears to have drained laterally and may also have reentered the regional subsurface groundwater system.