Since its discovery in 1840, the Ross seal (Ommatophoca rossii) has rarely been observed. The present thesis provides an overview of the biology of the Ross seal acquired from the literature available. However, the corresponding underwater vocalizations have not been described in detail before. The Southern Ocean is largely unaffected by anthropogenic noise. Therefore, it provides the ideal location for long-term underwater recordings as implemented with PALAOA (PerenniAL Acoustic Observatory in the Antarctic Ocean). This listening station, set up by the Alfred Wegener Institute for Polar and Marine Research (AWI) in 2005, is located on the Ekström Ice Shelf at Atka Bay, Antarctica, consisting of an array of hydrophones deployed through the ice shelf. PALAOA was designed to autonomously obtain year- round broadband (15 Hz - 96 kHz) and high resolution (up to 24bit) underwater recordings, while providing real-time data access. For analysis, these recordings were scanned visually and aurally to characterize 280 Ross seal calls in detail, and to count an additional 17 000 calls for seasonal and diurnal calling rates. The main result of this thesis is the differentiation of four distinct call types: three siren calls (High, Mid, Low) and the Whoosh. These call types can easily be discerned by their maximum and minimum frequencies. This detailed characterization of underwater call types provides the basis for further investigations on geographic variation within Ross seal vocalizations, and for the development of automated pattern recognition algorithms. It has been found that the annual acoustic presence of Ross seals at Atka Bay is the period between December and February only. The increase in calling rate in mid January matches recent satellite tagging experiments and is probably caused by the arrival of seals that were pelagic before. The striking drop at the end of January corresponds with the migration of most Ross seals northwards. During their presence at PALAOA, Ross seals show a clear diurnal calling pattern with peak calling rates around midnight, which correlates rather with daylight conditions than with tidal currents. The nocturnal peaks in calling rates of Ross seals are consistent with other Antarctic seal species.