Upsweep

Deep in the South Pacific Ocean, something has been making the same strange sound for over three decades. First detected in 1991 by NOAA’s underwater hydrophone network, the phenomenon known as “Upsweep” consists of a series of narrow-band ascending sounds that repeat in regular patterns, peaking in intensity during spring and autumn of each year. Despite extensive analysis by oceanographers and acoustic specialists, no definitive explanation has been established for what creates this persistent, enigmatic sound. Upsweep represents one of several unexplained acoustic phenomena detected in Earth’s oceans, reminding us that the deep sea remains one of the least understood environments on our planet.

Discovery and Characteristics

The Upsweep sound was first identified in August 1991 by the Pacific Marine Environmental Laboratory’s autonomous hydrophone array, originally deployed to monitor underwater seismic activity and marine mammal populations. The sound presents as a series of long, narrow-band upsweeping sounds lasting several seconds each, typically in the frequency range of around 1-6 Hz. What makes Upsweep particularly unusual is its consistency—it has been detected continuously since 1991, maintaining remarkably similar characteristics year after year. The sound is powerful enough to be detected by multiple sensors across the Pacific basin, indicating a significant energy source.

Seasonal Variation

One of the most intriguing aspects of Upsweep is its clear seasonal pattern. The sound is loudest and most frequent during spring and fall, with noticeable drops in intensity during winter and summer months. This seasonal variation suggests the source may be influenced by temperature changes, water circulation patterns, or some other environmentally-dependent process. The regularity of this pattern over more than 30 years rules out most one-time events and points toward an ongoing natural phenomenon that responds to predictable annual cycles.

Location and Volcanic Association

The source of Upsweep has been triangulated to a remote region of the South Pacific, approximately equidistant between New Zealand and South America at coordinates near 54°S, 140°W. This location places the sound source in a volcanically active area of the Pacific seafloor. The proximity to underwater volcanic features has led most researchers to favor geological explanations—specifically, that Upsweep may be produced by volcanic or hydrothermal activity on the ocean floor. Hot magma interacting with cold seawater, gas releases from underwater vents, or the movement of magma through submarine volcanic chambers could all potentially produce sounds matching Upsweep’s characteristics.

The Volcanic Hypothesis

The leading scientific explanation for Upsweep centers on underwater volcanic activity. Submarine volcanoes and hydrothermal vents are known to produce various sounds as superheated water and gases interact with the ocean. The seasonal variation might be explained by changes in water temperature and density affecting how sound propagates, or by seasonal changes in volcanic activity itself. However, no specific volcanic feature has been definitively identified as the source, and the extreme consistency of the sound over decades raises questions—most volcanic sounds are far more variable and episodic than Upsweep’s steady rhythm.

Related Ocean Sounds

Upsweep is one of several unexplained sounds detected by NOAA’s hydrophone network since the 1990s. The most famous is the “Bloop,” detected in 1997—an ultra-low-frequency sound that was eventually attributed to iceberg calving. Other unexplained sounds include “Slow Down,” a sound lasting about seven minutes that descended in frequency; “Julia,” a sound resembling someone saying the name; “Train,” which sounds like a distant locomotive; and “Whistle,” a consistent tone detected in the Pacific. Some of these have been explained, while others remain mysterious. Together, they demonstrate how little we understand about the acoustic environment of the deep ocean.

Alternative Theories

While volcanic activity remains the most accepted explanation, alternative theories have been proposed. Some researchers have suggested biological origins—perhaps a previously unknown species of large marine animal producing the sounds, though no known creature matches the acoustic profile. More speculative suggestions include classified military equipment, underwater industrial activity, or even more exotic explanations involving unknown geological processes or phenomena. Without direct observation of the source, none of these theories can be definitively ruled out.

The Mystery Persists

As of the 2020s, Upsweep continues its decades-long pattern, still detected, still unexplained, still rising from the depths of the South Pacific with clockwork regularity. The sound serves as a humbling reminder of how much remains unknown about our own planet. We have mapped the surface of Mars in greater detail than the deep ocean floor; we understand the composition of distant galaxies better than the acoustic environment of our own seas. Whatever produces Upsweep—volcano, vent, creature, or something entirely unexpected—it has been sending its signal upward through the water column for over 30 years, waiting for us to finally understand what it’s saying.

Sources

• Wikipedia search: “Upsweep”