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In recent years there is growing concern that the use of mid-frequency (1-10 kHz) tactical sonar may be causing atypical mass stranding of some species of whales. Beaked whales form a family of 21 species of mostly deep diving whales that range from 4 to 13 meters long with a characteristic `beak' similar to dolphins. The beaked whale species common in the Mediterranean Sea, the Cuvier's beaked whale (Ziphius cavirostris), seem uniquely vulnerable to the effects of mid-frequency sonar. These whales make deep dives (nearly 2000 m), typically lasting 20 to 30 minutes but as long as 85 minutes, and together with their minimal surface expression, make visual detection difficult. Although the detailed cause and effect of sonar-related whale strandings are still unknown and subject to intensive research, NURC has been developing a small towed system that not only detects deep-diving whales but also allows for real-time localization. The goal is to detect marine mammals in an area of acoustic experiments that involve high power sonar sources. The Compact Passive Acoustic Monitor (CPAM) lets researchers to monitor the environment with persistent data collection and results reporting. It uses advanced passive acoustic processing technology to capture and analyze very high frequency, short transient signals from marine mammals. A small volumetric array of four hydrophones together with depth and orientation sensors results in true three-dimensional direction finding. In this paper, we describe CPAM and some of the data obtained from engineering tests as well as a recent large-scale marine mammal survey cruise conducted in the Atlantic Ocean.