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Bloop

The Bloop is the name given to an ultra-low-frequency underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in the summer of 1997 by hydrophones including those in the Scotia Sea, with the source in the southern Pacific Ocean near Antarctica.^[1] This extremely loud noise, recorded by NOAA's hydrophone array over a range exceeding 5,000 kilometers, featured a broad spectrum signature with a distinctive rising frequency pattern, making it audible across vast distances in the ocean.^[1]^[2] Initially, the Bloop puzzled scientists and sparked speculation about its origins, including possibilities of large undiscovered marine animals or human activities, due to its intensity surpassing known biological sounds at the time.^[2] NOAA's Pacific Marine Environmental Laboratory (PMEL) analyzed the spectrogram, noting its similarity to other cryogenic signals but unmatched amplitude until comparable events were later observed.^[1] In 2005, researchers pinpointed the source near the Antarctic coast, between the Bransfield Strait and Cape Adare, through triangulation of hydrophone data.^[2]^[1] Subsequent investigations revealed the Bloop to be an icequake—a seismic event caused by the cracking and fracturing of a massive iceberg as it calved from an Antarctic glacier.^[2] This explanation was confirmed by comparing its acoustic profile to documented icequake sounds, such as the 2008 disintegration of iceberg A53a near South Georgia Island, which produced nearly identical broad-spectrum signals.^[1] The frequency of such icequake events has increased in recent decades, correlating with accelerated glacial melting due to global warming, highlighting the Bloop's role in monitoring Antarctic ice dynamics.^[2]

Discovery and Detection

NOAA Hydrophone Network

The NOAA Pacific Marine Environmental Laboratory (PMEL) deployed autonomous hydrophone arrays in the late 1990s to monitor underwater acoustic signals across vast ocean regions, utilizing repurposed military infrastructure for scientific purposes.^[3] These deployments built on the declassification of the U.S. Navy's Sound Surveillance System (SOSUS) in 1991, granting PMEL access to fixed hydrophone arrays in the North Pacific starting in October 1990.^[3] PMEL's autonomous systems complemented these by placing moored hydrophones in remote areas, such as the eastern equatorial Pacific since May 1996, to capture continuous data from the ocean soundscape.^[4] Central to the network's effectiveness is the Sound Fixing and Ranging (SOFAR) channel, a natural acoustic waveguide formed by the ocean's temperature and pressure gradients at depths of approximately 1,000 meters, where sound speeds are minimized.^[5] This channel enables low-frequency sounds (typically below 100 Hz) to propagate with cylindrical wavefronts over thousands of kilometers with minimal attenuation, far exceeding spherical spreading in shallower or deeper waters.^[6] As a result, signals can travel globally, allowing a sparse network of hydrophones to detect distant events by triangulating arrival times.^[6] The hydrophone arrays feature spacings of over 5,000 kilometers between stations, with individual hydrophones within each array positioned closer (on the order of hundreds of meters) to enable beamforming for directional analysis.^[7] They are sensitive to frequencies above 2 Hz and up to at least 100 Hz, capturing low-frequency acoustic energy efficiently within the SOFAR channel.^[6] Data are recorded autonomously by underwater moorings equipped with digital recorders, which store signals for later recovery via ship-based operations, providing long-term datasets without real-time transmission in the 1990s era.^[8] Historically, the precursor SOSUS network was established in the 1960s by the U.S. Navy for submarine detection but proved valuable for geophysical monitoring due to its global coverage.^[3] PMEL expanded its use in the 1990s to support NOAA's missions, including detecting submarine earthquakes (down to magnitude 2.5 via T-phase waves), tracking volcanic eruptions like those at Axial Seamount, and studying marine mammal vocalizations from species such as blue and sperm whales over hundreds of kilometers.^[6] This infrastructure facilitated detections like the 1997 Bloop signal across multiple Pacific sites.^[7]

The 1997 Recording Event

The Bloop sound was detected during the summer of 1997 by the U.S. National Oceanic and Atmospheric Administration (NOAA) through its extensive hydrophone array deployed in the southern Pacific Ocean.^[1] Researchers were actively monitoring the region for signs of underwater volcanic activity when the unusual signal was captured on multiple instruments.^[2] In 2005, the sound's origin was triangulated using data from hydrophones positioned more than 3,000 kilometers apart, allowing for precise localization to a point near Antarctica, between the Bransfield Strait and Cape Adare in the Scotia Sea, where it propagated efficiently through the SOFAR channel.^[2]^[1] Upon initial analysis, NOAA acousticians flagged the recording as highly anomalous due to its extraordinary high amplitude and ultra-low-frequency profile, which surpassed the intensity of documented marine mammal vocalizations.^[2] This prompted immediate archiving of the raw audio data for further study, marking the event as one of the most powerful unidentified underwater signals ever instrumented.^[1]

Acoustic Characteristics

Sound Profile and Spectrogram

The Bloop sound features a distinctive ultra-low-frequency waveform consisting of short-duration broad-band impulses typical of ice calving events. These pulses occurred sporadically, repeating over the course of the approximately one-minute recording captured in 1997.^[1] In spectrographic representations, the Bloop appears as a broad, irregular band spanning approximately 1-440 Hz, with energy concentrated in ultra-low frequencies, setting it apart from conventional seismic or marine mammal vocalizations through its unique visual pattern of energy distribution.^[9]^[10] Archival audio of the Bloop, released by NOAA, preserves the original recording sped up 16 times to render the ultra-low frequencies audible to humans, verifying its authenticity without artificial modifications.^[2]

Intensity, Frequency, and Propagation

The Bloop sound exhibited a broadband profile spanning approximately 1 to 440 Hz in its calving-like components.^[11] This range positioned it within the low-frequency spectrum detectable by NOAA's hydrophone network, distinguishing it from higher-frequency ocean noises while aligning with broadband impulsive signals typical of cryogenic events.^[1] In terms of intensity, the Bloop registered as one of the most powerful non-anthropogenic underwater sounds captured, with estimated source levels reaching up to 247 dB re 1 μPa at 1 m for comparable Antarctic icequake signals. These levels, calculated from received signals on multiple hydrophones, underscore its exceptional acoustic energy, far exceeding typical marine mammal vocalizations, which rarely surpass 190 dB. The sound's propagation was remarkable, traveling over 5,000 km across the southern Pacific Ocean with minimal attenuation, facilitated by the SOFAR channel's refractive properties arising from deep-water temperature and salinity gradients.^[1] This channel acts as a natural waveguide for low-frequency sounds, channeling them at mid-depths (around 1,000 m) where sound speed minima reduce spreading losses. Measurements involved adjusting received levels for geometric spreading (using spherical or cylindrical models depending on distance) and absorption coefficients specific to seawater, derived from hydrophone sensitivity calibrations at 250 Hz sampling rates.

Hypotheses on Origin

Biological Explanations

The initial hypothesis for the origin of the Bloop sound, recorded by the National Oceanic and Atmospheric Administration (NOAA) in 1997, posited it as a vocalization from a massive unidentified marine creature substantially larger than a blue whale, given its extraordinary amplitude that allowed detection across more than 5,000 kilometers in the Pacific Ocean.^[12] This speculation arose because the sound's intensity surpassed documented cetacean vocalizations, such as those of blue whales, which are the loudest known biological sources in the ocean.^[13] NOAA scientists initially considered biological sources among other possibilities, including whales or unknown sea creatures, without ruling out or confirming an animal origin in public releases from 1997 to 2001.^[2] Supporting arguments in the pre-2000s era emphasized the absence of any matching known species. This fueled interest among amateur enthusiasts and media outlets, drawing parallels to longstanding cryptozoological concepts of enormous sea monsters, such as the Cadborosaurus reported in Pacific folklore. Key proponents included NOAA geophysicist Christopher Fox, who noted the Bloop's spectrogram resembled marine animal signals, and marine biologist Phil Lobel of Boston University, both advocating for a biological interpretation in early discussions.^[13] However, biological explanations faced significant challenges, including the sound's ultra-low frequency range (peaking below 50 Hz) and brief duration of approximately one minute, which did not align with the sustained, repetitive patterns typical of animal communication for mating or navigation.^[1] Furthermore, extensive monitoring yielded no visual sightings, strandings, or biological traces to corroborate the presence of such a colossal organism, leading most NOAA researchers, including acoustics manager Bob Dziak, to dismiss the idea as unlikely from the outset.^[12]

Non-Biological Explanations

Prior to the establishment of the icequake consensus, researchers at the National Oceanic and Atmospheric Administration (NOAA) explored several non-biological explanations for the Bloop sound, focusing on abiotic sources that could generate such an ultra-low-frequency, high-amplitude acoustic event in the remote southern Pacific Ocean near Antarctica. These hypotheses were evaluated through analysis of the sound's spectrogram, propagation patterns, and regional geophysical data, but were ultimately dismissed due to mismatches with observed characteristics and lack of corroborating evidence.^[2]^[7] Seismic hypotheses posited that the Bloop could result from a distant earthquake or underwater volcanic eruption, given the sound's intensity and low-frequency profile, which superficially resembled seismic tremors recorded by hydrophone arrays. However, these ideas were ruled out because the Bloop lacked the characteristic pulsing or oscillatory nature of typical seismic events, and triangulation of the signal pinpointed its origin to a biologically inactive region near Antarctica rather than known seismic zones.^[2]^[7] Man-made possibilities were also considered, including submarine propulsion systems, underwater military exercises, or accidental explosions from shipping or fishing operations, as human activities occasionally produce comparable broadband noises detectable over long distances. These were dismissed due to the absence of corresponding military or navigational records in the area during the 1997 event, the irregular and non-repetitive pattern of the sound, and its incompatibility with known anthropogenic acoustic signatures from hydrophone data.^[2] Other geophysical mechanisms, such as underwater landslides or emissions from hydrothermal vents, were briefly hypothesized based on their potential to generate low-frequency rumbles in oceanic environments. Yet, these were excluded after comparative analysis showed that the Bloop's frequency range and duration did not align with documented events from similar regional features, where such sounds typically exhibit shorter bursts or higher-frequency components.^[2]^[7] These non-biological explanations were systematically assessed during NOAA's initial investigations from 1997 to 2003, as part of broader efforts to catalog equatorial Pacific acoustics using the U.S. Navy's Sound Surveillance System (SOSUS) hydrophones, before attention shifted toward cryospheric sources in subsequent years.^[7]

Icequake Consensus

Ice Calving Mechanisms

Ice calving refers to the mechanical breaking off of large chunks of ice from the terminus of tidewater glaciers or ice shelves, a process prevalent along Antarctic margins where ice meets the ocean. This detachment occurs when accumulated strain from gravitational forces, tidal flexing, and basal melting exceeds the ice's tensile strength, leading to fracture propagation along preexisting crevasses. The sudden release of elastic strain energy during calving generates seismic tremors and acoustic emissions that radiate into the surrounding seawater as pressure waves.^[1]^[14] The acoustic signals produced by ice calving arise primarily from the rapid fracturing and displacement of ice blocks, which create broadband, low-frequency pulses through explosive decompression and impact with the water column. In subaerial calving events—where ice falls from above the waterline—the initial crack propagation emits short-duration impulses dominated by frequencies below 200 Hz, peaking around 35-50 Hz, as the ice block impacts the sea surface and entrains air, amplifying the signal via bubble oscillations. Submarine calving, occurring below the waterline, generates similar low-frequency content (<0.1 kHz) but with less high-frequency noise, resulting from direct ice-water interactions and reduced air entrainment. These mechanisms parallel seismic rock bursts in mining but operate under cryogenic conditions, where ice's brittle behavior at temperatures near -20°C to -30°C favors impulsive energy release rather than ductile flow.^[14]^[15] In the case of the Bloop signal, detected by NOAA hydrophones in the summer of 1997, with its source triangulated to near the Bransfield Strait in the Southern Ocean adjacent to the Pacific, acoustic analysis identified its ultra-low-frequency profile as originating from large-scale ice calving events near the Bransfield Strait and South Shetland Islands, Antarctica. Surveys by NOAA's Pacific Marine Environmental Laboratory from 2005 to 2010 confirmed matches between the Bloop's broadband spectrum (1-440 Hz) and contemporary calving noises from fracturing icebergs, such as the tracked iceberg A53a, distinguishing it from biological or other abiotic sources through propagation patterns detectable over 5,000 km. This resolution highlighted calving as a dominant source of transient ocean noises in polar regions.^[1]^[16] Environmental drivers, particularly anthropogenic climate warming, have intensified ice calving dynamics in Antarctica by elevating air and ocean temperatures, which enhance surface melting, undercutting, and hydrostatic stress at glacier termini. This acceleration results in more frequent and voluminous calving episodes, thereby increasing the occurrence of associated ultra-low-frequency acoustic signals that propagate across ocean basins. For instance, West Antarctic ice shelves have exhibited heightened mass loss via calving since the late 20th century, correlating with regional warming trends of over 2°C. As of 2025, Antarctic sea ice has seen record lows, with the 2024 winter maximum at approximately 17.16 million km², contributing to heightened calving activity.^[17]^[18]^[19]

Ice Floe Dynamics

Although not the identified source of the Bloop, which matches ice calving profiles, ice floe ridging events arise from the collision and buckling of sea ice floes driven by wind or ocean current pressures in Antarctic waters. These interactions cause the grinding and deformation of ice, generating low-frequency rumbles that propagate into the underwater environment as broadband acoustic signals. Such sounds typically feature spectral peaks around 10 Hz, dominated by the mechanical failure of ice under compressive stress.^[20] Rubbing mechanisms involve shear friction between overlapping ice floes or between floes and the underlying seabed, producing sustained yet pulsed noises characterized by harmonic signatures from resonant vibrations. These friction-induced sounds extend up to approximately 50 Hz, reflecting the intermittent contact and sliding of ice surfaces during mobile pack ice conditions. In the Southern Ocean, where ice floe interactions are prevalent, these acoustics contribute significantly to the ambient noise landscape, often overlapping with bending or flexural modes of ice.^[21]^[22] The acoustic profile of ice floe dynamics bears similarity to the 1997 Bloop recording, as both exhibit ultra-low-frequency content and high amplitude capable of long-range detection. Antarctic sea ice extent in 1997 was relatively high for the early satellite era, fostering conditions for widespread ridging and enhanced floe interactions. These sounds travel efficiently through the SOFAR channel, enabling propagation over thousands of kilometers with minimal attenuation, akin to the Bloop's trans-Pacific detection pattern. Cryoseismic monitoring in the Palmer Peninsula region has documented analogous events, linking them to climate-influenced ice mobility rather than biological sources.^[23]^[24]^[25]

Cultural and Scientific Impact

Role in Cryptozoology

Following the public release of the Bloop audio recording by NOAA in 2001, the sound rapidly gained traction within cryptozoological circles, where it was interpreted as evidence of an enormous, unidentified marine creature. Enthusiasts speculated it originated from biological sources such as oversized cephalopods like giant squid, unknown species of baleen whales, or even colossal entities reminiscent of H.P. Lovecraft's Cthulhu, whose fictional sunken city of R'lyeh lies approximately 1,500 kilometers from the detection site in the South Pacific.^[26]^[27] This pseudoscientific narrative was amplified through various media outlets during the early 2000s, appearing in online forums, speculative literature, and short documentaries that portrayed the Bloop as a harbinger of deep-sea monstrosities. For instance, the 2016 short film The Bloop by Cara Cusumano explored its enigmatic allure through interviews with NOAA scientists, while creepypasta stories on internet fiction platforms wove it into horror tales of awakening leviathans, often overlooking or dismissing the agency's preliminary 2005 attribution to non-biological phenomena.^[28]^[26] The myth's endurance persisted into the 2010s and beyond, even as further clarifications emerged, with the Bloop frequently invoked in podcasts, video games, and viral content as an emblem of oceanic unknowns. Cultural interest peaked between 1997 and 2008 amid widespread speculation, but saw revivals in the 2020s through discussions tying unresolved sea mysteries to broader environmental narratives, sustaining its status as a cryptozoological icon.^[29]^[27]

Implications for Ocean Acoustics Research

The identification of the Bloop as an icequake in 2005 by NOAA's Pacific Marine Environmental Laboratory (PMEL) marked a pivotal advancement in ocean acoustics, enabling researchers to refine techniques for distinguishing geophysical sounds from biological ones through detailed spectrogram analysis of broadband signals spanning 1-440 Hz.^[1] This breakthrough, achieved using the extensive hydrophone arrays of the NOAA Acoustics Program, demonstrated the capability to localize distant events over 5,000 km, such as those originating from Antarctic regions like the Bransfield Straits to the [Ross Sea](/page/Ross Sea).^[2] Post-identification efforts have integrated these acoustic signatures into broader monitoring frameworks, enhancing the precision of passive acoustic recorders for tracking iceberg calving and fracturing.^[1] In terms of climate monitoring, Bloop-like icequake sounds have emerged as key indicators of Antarctic ice loss, with their increasing frequency attributed to global warming-induced glacier instability.^[2] NOAA's ongoing hydrophone deployments have captured similar ultra-low-frequency events in the southern and Atlantic Oceans in recent years, correlating them with accelerated iceberg disintegration and providing data to assess cryospheric changes.^[30] These acoustics contribute to environmental studies by revealing patterns in ice dynamics, such as harmonic tremors during calving, which inform models of sea-level rise and polar ecosystem impacts.^[1] The Bloop investigation has bolstered broader ocean acoustics research by underscoring the value of long-term hydrophone networks, now part of NOAA's ocean noise reference stations that span U.S. waters and beyond.^[2] This has facilitated the detection of thousands of ice-related events annually, filling gaps in our understanding of the approximately 73% of the ocean floor that remains unmapped as of 2025.^[1]^[31]

References

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The broad spectrum sounds recorded in the summer of 1997 are consistent with icequakes generated by large icebergs as they crack and fracture.Missing: underwater | Show results with:underwater
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