20 Deep Ocean Signals No One Can Identify

Scientists have recorded mysterious signals from the deep ocean that remain unexplained despite decades of research.

Sophia Zapanta
November 20, 2025
5 min read

20 Deep Ocean Signals No One Can Identify — Jeremy Bishop on Pexels

The deep ocean is a largely unexplored environment where unusual sounds and signals occasionally emerge. Some of these signals resemble patterns from unknown marine life, natural geological events, or even human-made sources, but many remain a complete mystery. This list explores 20 of the most intriguing deep ocean signals that scientists have yet to identify.

1. 1. Bloop (1997)

National Oceanic and Atmospheric Administration on Wikimedia Commons

The Bloop was a powerful, ultra-low-frequency sound recorded in the South Pacific. Its amplitude was far louder than any known marine animal. Initially, some speculated it came from a giant, unknown creature. Later analysis suggested it might be icequakes, though the sound’s origin is still debated.

2. 2. Upsweep

Upsweep is a long, continuous sound detected in the Pacific Ocean. It rises and falls in pitch over hours, repeating seasonally. Scientists have yet to pinpoint the exact source. Its consistency hints at natural geological or hydrothermal activity.

3. 3. Julia

Julia is a series of tonal sounds recorded near Antarctica. The sounds travel thousands of miles underwater, indicating a powerful source. Despite detailed analysis, no known species or geological event matches it. Its origin remains one of the ocean’s most mysterious acoustic phenomena.

4. 4. Slow Down

This sound gradually decreases in frequency over minutes. It was detected in the South Pacific in 1997. The source is unclear, but some think it may be related to ice movement or seabed activity. Its unusual pattern puzzles marine researchers.

5. 5. Train

Kabelleger / David Gubler on Wikimedia Commons

The train produces rhythmic, train-like pulses underwater. The sounds were recorded near the Pacific-Antarctic Ridge. No marine species are known to generate this pattern. Some scientists suggest it may be volcanic or tectonic activity.

6. 6. Whistle

NASA/JPL-Caltech/University of Iowa on Wikimedia Commons

This is a long, high-pitched sound heard sporadically in the North Pacific. It resembles marine mammal calls but is far louder. The source has never been confirmed. The signal’s mystery remains unresolved.

7. 7. Bio-duck

First recorded in the Southern Ocean, the bio-duck sound resembles a duck quacking underwater. It occurs in seasonal patterns. Its source was recently linked to Antarctic minke whales. However, the full behavior and range of the sound are still poorly understood.

8. 8. Upsweep 2

A variation of the original Upsweep, this sound is irregular but maintains a rising pitch. It has been detected in different Pacific regions. Researchers are unsure if it’s geological or biological. Its unpredictable nature adds to its mystery.

9. 9. Bloop Twin

Similar to the Bloop, this sound appears in the same frequency range but is weaker. It was recorded a few years after the original Bloop. Its connection to icequakes or unknown life is debated. Scientists have yet to draw firm conclusions.

10. 10. The Train 2

A repeat of rhythmic pulses resembling the first Train sound. Recorded in nearby locations, it follows a slightly different pattern. No clear origin has been confirmed. The repeating signals suggest persistent natural processes.

11. 11. Ping

Short, high-frequency sounds recorded globally. Pings are brief and isolated. The cause could be marine animals, underwater volcanism, or human-made sonar. Their random occurrence keeps them unexplained.

12. 12. Upsweep 3

This is a deeper variation of the Upsweep sound. It has a lower frequency and a longer duration. Scientists have yet to determine its source, but its scale indicates a powerful natural phenomenon.

13. 13. The Hum

Low-frequency humming is heard in deep trenches. It travels vast distances underwater. The exact origin is unknown, but it may relate to tectonic activity. Its consistency remains a puzzle to researchers.

14. 14. The Croak

A strange, short pulsing sound was detected in the North Atlantic. It resembles a croaking noise but underwater. No animal or geological source has been confirmed. The signal appears sporadically and unpredictably.

15. 15. Chirp

A rapid series of pulses resembling bird chirps was detected underwater. Recorded in the Pacific, its source is unknown. It may be biological, geological, or mechanical. Its irregular pattern makes it difficult to classify.

16. 16. Moan

A deep, sustained sound was recorded in the Mariana Trench. The sound lasted several minutes and carried far. Scientists have not identified its source, but its eerie tone suggests either deep-sea currents or unknown marine life.

17. 17. Trumpet

A loud, trumpet-like call was detected in Antarctic waters. Its amplitude suggested a large source. Researchers have yet to find a matching creature or process, and the sound’s sudden appearance is mysterious.

18. 18. Clicking Pulse

Rapid sequences of clicks were recorded intermittently. They traveled long distances through the ocean. The source remains uncertain, though some suggest geological shifts, and the unusual pattern defies current classification.

19. 19. Deep Water Boom

A massive, low-frequency explosion-like sound was detected in the Atlantic, which caused a brief spike in underwater acoustic readings. Scientists were unsure if it was volcanic, ice-related, or biological. Its rarity keeps it unexplained.

20. 20. Sea Monster Call

This is an informal name for an unidentified deep-sea sound with varying pitch. Recorded in the Pacific, it resembles a large animal vocalization. No species has been identified. Its mysterious origin fuels speculation about unknown ocean life.