An illustrative hydrophone deployment in Baffin Bay provides a practical account of the information needed to interpret underwater recordings. The essay follows calibration, clocks, depth, geometry, vessel activity, and handling noise from preparation through classification. It explains why a plausible narwhal signal can remain uncertain, why masking can prevent detection, and why a recording without confirmed calls cannot establish animal absence. Published evidence on changing ocean structure supplies context without allowing one deployment to attribute regional ecological change. The discussion extends to sensitive locations, inadvertently recorded voices, access agreements, and the processing history of representative clips. Its central implication is that a reusable acoustic file preserves uncertainty and governance alongside the sound, rather than presenting an attractive recording as a census or a documented expedition.