Monitoring subsea temperature variations is essential for capturing dynamic physical processes, including mesoscale eddies, wind-driven upwelling and downwelling, internal wave propagation, and turbulent mixing. These phenomena strongly influence nutrient distribution and biological productivity within marine ecosystems. However, traditional in situ measurements often fail to resolve fine-scale thermal fluctuations due to limited sampling density. To address this limitation, Distributed Acoustic Sensing (DAS) offers a transformative solution by leveraging existing fiber-optic infrastructure to enable continuous, high-resolution monitoring of the subsea environment. Nevertheless, low-frequency (LF) DAS signals are influenced by multiple factors, including mechanical cable vibrations and deformation, thermo-optic effects, and optical noise, which complicate their interpretation.Here, we evaluate the potential and limitations of DAS for long-range temperature measurements by characterizing the LF-DAS response to subsea temperature variations and optimizing these signals across timescales from days to seasons. The results show that DAS strain and temperature are highly coherent (>0.5) at frequencies below 10 cycles per day. After denoising, DAS strain variations correlate well with temperature changes ranging from 0.4 to 10 K, although discrepancies between channels emerge at ultra-low frequencies. These signals are likely influenced by optical noise and amplified during rapid temperature changes, but can be mitigated through spatial averaging. With preliminary processing, DAS can resolve temperature fluctuations below 0.1 K, achieving meter-scale spatial resolution and minute-scale temporal resolution. These results demonstrate that DAS provides a powerful approach for observing subsea temperature variability, offering new insights into ocean dynamics through unprecedented spatiotemporal resolution.

Distributed Acoustic Sensing (DAS) is a photonics technology converting seafloor telecommunications and optical fiber cables into dense arrays of strain sensors, allowing to monitor various oceanic physical processes. Yet, several applications are hindered by the limited knowledge of the transfer function between geophysical variables and DAS measurements. This study investigates the quantitative relationship between surface gravity DAS-recorded wave-generated strain signals along the seafloor and the pressure at a colocated sensor. A remarkable linear correlation is found over various sea conditions allowing us to reliably determine significant wave heights from DAS data. Utilizing linear wave potential theory, we derive an analytical transfer function linking cable deformation and wave kinematic parameters. This transfer function provides a first quantification of the effects related to surface gravity waves and fiber responses. Our results validate DAS's potential for real-time reconstruction of the surface gravity wave spectrum over extended coastal areas. It also enables the estimation of waves hydraulic parameters at depth without the need from offshore deployments.

The KM3NeT Collaboration is building an underwater neutrino observatory at the bottom of the Mediterranean Sea, consisting of two neutrino telescopes, both composed of a three-dimensional array of light detectors, known as digital optical modules. Each digital optical module contains a set of 31 three-inch photomultiplier tubes distributed over the surface of a 0.44 m diameter pressure-resistant glass sphere. The module also includes calibration instruments and electronics for power, readout, and data acquisition. The power board was developed to supply power to all the elements of the digital optical module. The design of the power board began in 2013, and ten prototypes were produced and tested. After an exhaustive validation process in various laboratories within the KM3NeT Collaboration, a mass production batch began, resulting in the construction of over 1200 power boards so far. These boards were integrated in the digital optical modules that have already been produced and deployed, which total 828 as of October 2023. In 2017, an upgrade of the power board, to increase reliability and efficiency, was initiated. The validation of a pre-production series has been completed, and a production batch of 800 upgraded boards is currently underway. This paper describes the design, architecture, upgrade, validation, and production of the power board, including the reliability studies and tests conducted to ensure safe operation at the bottom of the Mediterranean Sea throughout the observatory's lifespan.

We present the result of a cross-disciplinary collaboration between the California College of the Arts, San Francisco and the KM3NeT Collaboration. The project involved creating an analog sound producing instrument which was installed within a standard KM3NeT pressure resistant glass sphere housing along with sound and video recording equipment. The instrument, titled "Bathysphere", was deployed in the sea for the first time adjacent to the ORCA array of KM3NeT off the coast of Toulon, France on September 23, 2021. One outcome of this project is the video work "Below the Surface". It incorporates sound and video recorded from within the "Bathysphere" as it floated on the surface of the Mediterranean and then as it dived down to 300 m depth. As the "Bathysphere" dived, the analog instrument it contained quietened. The sound in the dive portion of the video is created from the sonification of data from the KM3NeT array that was recorded during the deployment of the "Bathysphere". "Below the Surface" highlights the extraordinary environment in which the KM3NeT array is being created and operates. The data sonification illustrates the potential of this method of data representation to connect with viewers in a deeply physical way and offers new perspectives on the data collected by KM3NeT.

Temperature is an essential oceanographic variable (EOV) that still today remains coarsely resolved below the surface and near the seafloor. Here, we gather evidence to confirm that Distributed Acoustic Sensing (DAS) technology can convert tens of kilometer-long seafloor fiber-optic telecommunication cables into dense arrays of temperature anomaly sensors having millikelvin (mK) sensitivity, thus allowing to monitor oceanic processes such as internal waves and upwelling with unprecedented detail. Notably, we report high-resolution observations of highly coherent near-inertial and super-inertial internal waves in the NW Mediterranean sea, offshore of Toulon, France, having spatial extents of a few kilometers and producing maximum thermal anomalies of more than 5 K at maximum absolute rates of more than 1 K/h. We validate our observations with in-situ oceanographic sensors and an alternative optical fiber sensing technology. Currently, DAS only provides temperature changes estimates, however practical solutions are outlined to obtain continuous absolute temperature measurements with DAS at the seafloor. Our observations grant key advantages to DAS over established temperature sensors, showing its transformative potential for the description of seafloor temperature fluctuations over an extended range of spatial and temporal scales, as well as for the understanding of the evolution of the ocean in a broad sense (e.g. physical and ecological). Diverse ocean-oriented fields could benefit from the potential applications of this fast-developing technology.

Ocean water temperature measurements are fundamental to atmospheric and ocean sciences. Obtaining them, however, often comes along with major experimental and logistic challenges. Except for the uppermost ocean surface temperature, which can be measured from satellites, temperature data of the ocean is often poorly sampled or nonexistent, especially in deep-water regions. Although Distributed Acoustic Sensing (DAS) technology has become popular because its high sensitivity to strains and mechanical vibrations, our work focuses on its usage on tens-of-kilometer-long underwater fibre-optic (FO) telecommunication cables to measure temperature anomalies at the seafloor at millikelvin (mK) sensitivity. This is possible because of the lack of dominant strain signals at frequencies less than about ∼1 mHz, as well as the poor coupling of the fibre with these signals while remaining highly sensitive to slow ambient temperature variations that locally affect its optical path length. DAS allows us to observe significant temperature anomalies at the continental shelf and slope of the Mediterranean sea, South of Toulon, France over periods of several days, with variability remaining relatively low at the deep ocean. By means of this approach, oceanic processes such as near-inertial internal waves and upwelling can be monitored at unprecedented detail.Our observations are validated with oceanographic in-situ sensors and alternative Distributed Fibre Optic Sensing (DFOS) technologies established for temperature sensing. We outline key advantages of DAS thermometry over the aforementioned sensors in terms of spatial coverage, sensitivity, versatility and highest attainable frequency. At the current state of the art, DAS can only measure temperature anomalies as opposed to absolute temperature, a drawback that could be compensated via single temperature calibration measurements.

Plastic and microplastic pollutions are known to be widespread across the planet in all types of environments. However, relatively little about microplastic quantities in the deeper areas of the oceans is known, due to the difficulty to reach these environments. In this work, we present an investigation of microplastic (<5 mm) distribution performed in the bottom sediments of the abyssal plain off the coast and the canyon of Toulon (France). Four samples of deep-sea sediment were collected at the depth of 2443 m during the sea operations carried out by the French oceanographic cruises for the KM3NeT project. The chemical and physical characterisation of the sediment was carried out, and items were extracted from sediments by density separation and analysed by optical microscope and µRaman spectroscopy. Results show microplastics in the deep-sea sediments with a concentration of about 80 particles L−1, confirming the hypothesis of microplastics spread to abyssal sediments in the Mediterranean Sea.

KM3NeT/ORCA is a dense array that constitutes the low-energy branch of the KM3NeT project with the main goal of resolving the question of the neutrino mass ordering. At present, the KM3NeT/ORCA Phase 1 has already been deployed, which means that six out of the planned 115 detection lines are operational. Even with this limited configuration, neutrino oscillations can already be measured and studied. In this contribution, the sensitivity to the neutrino Non-Standard Interactions (NSI) parameter $\epsilon_{\mu \tau}$ using the current stage of the KM3NeT/ORCA detector together with the projections for the final configuration are presented.

. The KM3NeT infrastructure consists of two deep-sea neutrino telescopes being deployed in the Mediterranean Sea. The telescopes will detect extraterrestrial and atmospheric neutrinos by means of the incident photons induced by the passage of relativistic charged particles through the seawater as a consequence of a neutrino interaction. The telescopes are configured in a three-dimensional grid of digital optical modules, each hosting 31 photomultipliers. The photomultiplier signals produced by the incident Cherenkov photons are converted into digital information consisting of the integrated pulse duration and the time at which it surpasses a chosen threshold. The digitization is done by means of time to digital converters (TDCs) embedded in the field programmable gate array of the central logic board. Subsequently, a state machine formats the acquired data for its transmission to shore. We present the architecture and performance of the front-end firmware consisting of the TDCs and the state machine. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JATIS.7.1.016001]

An anomalous flux of neutrinos produced in hypothetical annihilations or decays of dark matter inside a source would produce a signal observable with neutrino telescopes. As suggested by observations, a conspicuous amount of dark matter is believed to accumulate in the centre of our Galaxy, which is in neat visibility for the Mediterranean underwater telescopes ANTARES and KM3NeT. Searches have been conducted with a maximum likelihood method to identify the presence of a dark matter signature in the neutrino flux measured by ANTARES. Results of all-flavour searches for WIMPs with masses from 50 GeV/c$^2$ up to 100 TeV/c$^2$ over the whole operation period from 2007 to 2020 are presented here. Alternative scenarios which propose a dark matter candidate in the heavy sector extensions of the Standard Model would produce a clear signature in the ANTARES telescope, that can exploit its view of the Galactic Centre up to high energies. The presentation of Galactic Centre searches is completed with ongoing analyses and future potential of the KM3NeT telescope, in phased construction in the Mediterranean Sea.

The status and physics performance of the KM3NeT neutrino telescope in the Mediterranean Sea neutrino are presented.

The KM3NeT research infrastructure being built at the bottom of the Mediterranean Sea will host water-Cherenkov telescopes for the detection of cosmic neutrinos. The neutrino telescopes will consist of large volume three-dimensional grids of optical modules to detect the Cherenkov light from charged particles produced by neutrino-induced interactions. Each optical module houses 31 3-inch photomultiplier tubes, instrumentation for calibration of the photomultiplier signal and positioning of the optical module and all associated electronics boards. By design, the total electrical power consumption of an optical module has been capped at seven watts. This paper presents an overview of the front-end and readout electronics system inside the optical module, which has been designed for a 1~ns synchronization between the clocks of all optical modules in the grid during a life time of at least 20 years.

KM3NeT, currently under construction in the abysses of the Mediterranean Sea, is a distributed research infrastructure that will host a km3-scale neutrino telescope (ARCA) for high-energy neutrino astronomy, and a megaton scale detector (ORCA) for neutrino oscillation studies of atmospheric neutrinos. ORCA is optimised for a measurement of the mass hierarchy, providing a sensitivity of 3{\sigma} after 3-4 years. It will also measure the atmospheric mixing parameters $\Delta m_{32}^2$ and $\theta_{23}$ with a precision comparable to the NOvA and T2K experiments using both the muon neutrino disappearance and tau neutrino appearance channels. It will provide a measurement of the tau neutrino appearance rate with better than 10% precision, a crucial ingredient for tests of unitarity. It will probe the octant of the mixing angle $\theta_{23}$ via matter resonance effects on neutrinos and antineutrinos crossing the core and mantle, which are largely independent on the CP phase. The observation of neutrino oscillations over a wide range of baselines and energies will provide broad sensitivity to new physics such as non-standard neutrino interactions (NSI) and sterile neutrinos.

The deep ocean is the largest and least known ecosystem on Earth. It hosts numerous pelagic organisms, most of which are able to emit light. Here we present a unique data set consisting of a 2.5-year long record of light emission by deep-sea pelagic organisms, measured from December 2007 to June 2010 at the ANTARES underwater neutrino telescope in the deep NW Mediterranean Sea, jointly with synchronous hydrological records. This is the longest continuous time-series of deep-sea bioluminescence ever recorded. Our record reveals several weeks long, seasonal bioluminescence blooms with light intensity up to two orders of magnitude higher than background values, which correlate to changes in the properties of deep waters. Such changes are triggered by the winter cooling and evaporation experienced by the upper ocean layer in the Gulf of Lion that leads to the formation and subsequent sinking of dense water through a process known as "open-sea convection". It episodically renews the deep water of the study area and conveys fresh organic matter that fuels the deep ecosystems. Luminous bacteria most likely are the main contributors to the observed deep-sea bioluminescence blooms. Our observations demonstrate a consistent and rapid connection between deep open-sea convection and bathypelagic biological activity, as expressed by bioluminescence. In a setting where dense water formation events are likely to decline under global warming scenarios enhancing ocean stratification, in situ observatories become essential as environmental sentinels for the monitoring and understanding of deep-sea ecosystem shifts.

The ANTARES deep sea neutrino telescope has acquired over four years of high quality data. This data has been used to measure the oscillation parameters of atmospheric neutrinos and also to search for neutrinos of a non-terrestrial origin. Competitive upper limits on the fluxes of neutrinos from dark matter annihilation in the Sun, a variety of Galactic and extra-galactic sources, both steady and transient, are presented.

Various aspects of the construction, operation and calibration of the recently completed deep-sea ANTARES neutrino telescope are described. Some first results obtained with a partial five line configuration are presented, including depth dependence of the atmospheric muon rate, the search for point-like cosmic neutrino sources and the search for dark matter annihilation in the Sun.

The status and future plans of the ongoing underwater neutrino telescopes projects ANTARES, BAIKAL, NEMO, NESTOR and KM3NeT are presented.

A search for single sneutrino production under the assumption that R-parity is violated via a single dominant LLĒ coupling is presented. This search considers the process eγ → ν̃ℓ and is performed using the data collected by the ALEPH detector at centre-of-mass energies from 189 GeV up to 209 GeV corresponding to an integrated luminosity of 637.1 pb-1. The numbers of observed candidate events are in agreement with Standard Model expectations and 95% confidence level upper limits on five of the LLĒ couplings are given as a function of the assumed sneutrino mass.