On 29 August 1991, NOAA/PMEL began collecting ocean acoustic signals from the U.S. Navy's Sound Surveillance System (SOSUS) in the northeast Pacific Ocean. SOSUS consists of linear arrays of underwater hydrophones deployed on the seafloor within the sound channel that are directly cabled to shore. The NOAA/PMEL-developed system digitizes the acoustic signals directly from the underwater cables and passes the information via the phone line to NOAA laboratories in Newport, OR for analysis. The hydroacoustic method allows small submarine earthquakes (mb>1.8) to be routinely detected and located throughout the North Pacific. The primary advantages of this method are the large areal coverage from relatively few sensors and the low detection threshold and improved accuracy of the derived epicenter locations relative to land-based systems. Several significant discoveries have been made using the hydroacoustic method over the past 5 years, including (1) a better understanding of the distribution of seismicity in the northeast Pacific; (2) detection of volcanic activity and magma dike injection supported by extensive field observations; (3) detection of periodic volcanic tremor; and (4) the discovery of a band of low-level seismicity within the Gorda plate that may offer a precursor to subduction zone earthquakes.