English: This sound was present when PMEL began recording SOSUS in August, 1991. It consists of a long train of narrow-band upsweeping sounds of several seconds duration each. The source level is high enough to be recorded throughout the Pacific. It appears to be seasonal, generally reaching peaks in spring and fall, but it is unclear whether this is due to changes in the source or seasonal changes in the propagation environment. The source can be roughly located at 54o S, 140oW, near the location of inferred volcanic seismicity, but the origin of the sound is unresolved. The overall source level has been declining since 1991 but the sounds can still be detected on NOAA's equatorial autonomous hydrophone arrays.