Following the successful use of the U.S. Navy's SOSUS for monitoring low-level seismicity in the northeast Pacific, NOAA developed a strategy for monitoring remote areas of the world ocean not covered by fixed hydrophone arrays. This strategy requires the deployment of autonomous, hydrophone moorings able to record low-frequency acoustic energy in the SOFAR channel over extended periods. The instrument includes a single ceramic hydrophone, filter/amplifier stage designed to prewhiten the ambient noise spectrum, accurate clock (1 s/yr), and processor modified from off-the-shelf hardware. The instrument is capable of recording at 100 Hz (1 to 40-Hz bandpass) for periods of up to 1 year and is designed to be serviced at sea. A 2-month field test was conducted in the northeast Pacific during May--July 1995 to allow a direct comparison of the method with existing SOSUS arrays and other seismic experiments along the Juan de Fuca Ridge. The results indicate that the sensitivity of the moored systems is comparable to SOSUS detections. In May 1996, the array was deployed in the eastern equatorial Pacific to begin long-term monitoring of the East Pacific Rise between 20N and 20S, the Galapagos Ridge, and the Mid-America Trench.