Passive monitoring of underwater ambient noise for oceanographic applications has been underutilized. Underwater ambient sound contains quantifiable information about the sound source. In the frequency band from 500 Hz to 50 kHz, dominant sources of underwater sound include the sound of breaking waves and rainfall. The source of breaking waves is the local wind speed, allowing the amplitude of the ambient sound due to breaking waves to be very closely correlated with wind speed. The other dominant geophysical sound source is rainfall. This signal can also be quantified to monitor rainfall rate. The passive aquatic listener (PAL) is an adaptive low-duty-cycle recorder designed to monitor the background sound field in the marine environment. This signal is associated with the geophysical sound sources with longer time scales (from minutes to hours) and is used to quantify wind speed and rainfall rate. Ship passages and calling bouts lasting many minutes from marine mammal are also considered acoustic events, detectable using PALs that are also capable of detecting short transient sounds (bangs, clicks, and short calls). Identification and validation of these transient sounds are made by selectively preserving short acoustic samples of the sounds. This chapter describes the PAL instrument, the data collected, the acoustic processing of acoustic data, and the interpretation of data. Applications of these data to produce quantitative records of rainfall rate are also presented.