P-ONE: Pacific Ocean Neutrino Telescope

• Pacific Ocean Neutrino Experiment (P-ONE) is a deep-sea water-Cherenkov neutrino telescope concept in the Cascadia Basin designed to detect high-energy astrophysical neutrinos using Cherenkov light.
• It leverages the NEPTUNE observatory infrastructure and results from pathfinder missions like STRAW and STRAW-b for real-time data acquisition, calibration, and environmental monitoring.
• The design emphasizes complementary global network coordination, innovative detector architecture, and interdisciplinary oceanographic applications to enhance its scientific reach.

Pacific Ocean Neutrino Experiment (P-ONE) is a proposed deep-sea water-Cherenkov neutrino telescope for the Cascadia Basin off Vancouver Island, Canada, designed to instrument a volume beginning near the seafloor at 2660 m depth and to scale to a multi-cubic-kilometer observatory. The project is defined as much by its scientific scope as by its infrastructure model: it is the first large-volume neutrino-telescope concept to be built around an existing deep-ocean cabled observatory, NEPTUNE, operated by Ocean Networks Canada (ONC), which provides shore power, communications, and data services. Its design evolution has been driven by staged pathfinder missions—STRAW and STRAW-b—and by later subsystem studies on optical calibration, acoustic positioning, and long-term fouling at the site (Resconi et al., 2021, Holzapfel et al., 2023).

1. Scientific rationale and conceptual framework

P-ONE is motivated by the low flux of astrophysical high-energy neutrinos and the resulting need for substantially larger effective volumes and exposure than those of the current generation of telescopes. The design studies frame the instrument around astrophysical neutrinos above TeV energies and up to the PeV scale, with particular emphasis on long, near-horizontal muon tracks, while also retaining sensitivity to cascades and to all three flavors. The scientific program in the concept papers includes measurements of the diffuse astrophysical flux and its flavor composition, time-integrated and time-dependent searches for steady and flaring point sources, real-time transient alerting and follow-up, improved acceptance to horizontal tracks and cascades, tau-neutrino detection strategies, and contributions to high-energy neutrino–nucleon cross-section constraints (Resconi et al., 2021, Agostini et al., 2020).

Like other water-Cherenkov neutrino telescopes, P-ONE relies on the detection of Cherenkov light from charged secondaries produced in neutrino interactions in water. The underlying geometry is expressed in the usual relation

$\cos\theta_C = \frac{1}{\beta n},$

with $n$ the refractive index of seawater and $\beta = v/c$. In the design papers, expected event yields are written in terms of the effective area,

$$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$

and diffuse-flux studies use the standard parameterization

$$\Phi(E)=\Phi_0\left(\frac{E}{E_0}\right)^{-\gamma}.$$

These expressions are not specific to P-ONE, but in the P-ONE context they formalize the dependence of sensitivity on array geometry, medium properties, and sky coverage (Resconi et al., 2021, Henningsen et al., 2022).

A central strategic role of P-ONE is complementarity. The project is positioned as a Northern Pacific node in a global network with IceCube, KM3NeT, and Baikal-GVD, and the 2021 concept paper links it explicitly to PLE$\nu$M. In that network context, coordinated operations in the Southern Hemisphere can improve discovery potential by up to three orders of magnitude compared to a single telescope, with improved time-to-discovery on multi-year timescales. This network logic is integral to the P-ONE case for diffuse flux measurements, source identification, and multimessenger alerting rather than an ancillary consideration (Resconi et al., 2021).

2. Cascadia Basin site and ONC infrastructure

The detector site is the Cascadia Basin node of the NEPTUNE observatory, off Vancouver Island, Canada, at a depth of 2660 m. The site is repeatedly described as a relatively flat, calm sedimentary or abyssal-plain environment connected to shore by ONC’s cabled backbone. Oceanographic conditions quoted in the design overview include weak currents of approximately $3$–$7$ cm/s, water temperature of about $2^\circ$C, and salinity $3.482\% \pm 0.001$. These parameters matter directly for mooring mechanics, optical backgrounds, and long-term operations (Resconi et al., 2021, Henningsen et al., 2022).

NEPTUNE supplies the enabling infrastructure. The observatory is an 800 km cabled loop with five powered and data nodes, up to approximately 8 kW per node, data links up to approximately 4 Gbit/s, and 17 primary junction boxes. The concept and pathfinder papers emphasize that P-ONE leverages this existing power-plus-fiber system for deployment, communications, public data delivery, and scheduled maintenance. Pathfinder lines were connected through mini junction boxes to NEPTUNE hardware, and STRAW-b streamed data into ONC’s Oceans 2.0/3.0 databases using ONC data-product conventions and HDF5 formats (Resconi et al., 2021, Holzapfel et al., 2023).

This infrastructure model is one of P-ONE’s defining differences from other large neutrino telescopes. The 2021 concept paper notes underwater-mateable connectors with failure below $n$0 over about 10 years and a maintenance-cruise success rate of about $n$1, while the pathfinder papers show that continuous long-baseline environmental monitoring, remote DAQ control, and public archiving are already operational at the site. A plausible implication is that P-ONE’s technical risk is tied less to shore connection and more to detector-line engineering, calibration, and long-term survivability of deployed components (Resconi et al., 2021, Holzapfel et al., 2023).

3. Detector architecture, timing, and calibration systems

The full-scale vision presented in the concept studies consists of 7 clusters, each with 10 mooring lines, and each line carrying 20 optical modules of two types: optical receiver modules and calibration modules. A one-kilometer-tall line is the reference vertical scale. The 2021 concept paper describes a prototype line with 15 optical modules plus 5 optical/calibration modules, whereas the later optical-calibration paper describes the first line, P-ONE-1, as carrying 20 instruments: 18 P-OMs and two 17-inch P-CALs. This suggests an evolving prototype configuration within a stable overall architecture of kilometer-scale instrumented moorings (Resconi et al., 2021, Agostini et al., 10 Mar 2026).

The optical receiver concept is multi-PMT. Each digital optical module is described as hosting an array of small PMTs, with 3–3.5 inch candidates under evaluation in the 2021 paper, arranged in a “fly’s eye” hemispherical geometry. Connector-less cable integration is emphasized there as a means to reduce leak risk and simplify assembly. Timing distribution follows the White Rabbit protocol, with sub-ns synchronization via central FPGA logic, and the DAQ concept is explicitly FPGA-based with in-situ machine-learning filtering to suppress optical background while respecting ONC bandwidth and power constraints (Resconi et al., 2021).

Calibration is treated as a detector-defining subsystem rather than an accessory. Earlier P-ONE papers build on POCAM-style isotropic calibration sources and on STRAW’s multi-wavelength optical characterization; the 2026 optical-calibration paper extends this into production hardware for P-ONE-1. That paper reports 330 directional light pulsers produced, of which 318 passed quality control, and two isotropic 17-inch P-CALs. Across $n$2–$n$3 nm, the developed driver circuits achieve emission intensities up to $n$4 photons and pulse widths as small as $n$5 ns. The optimized P-CAL achieves a simulated isotropy grade of $n$6 across the full $n$7 solid angle and includes self-monitoring sensors for pulse-by-pulse normalization. In the same design, each P-OM hemisphere hosts 16 directional flashers in up, down, and sideways orientations, and each P-CAL hemisphere hosts five isotropic pulser channels plus directional sources and a camera (Agostini et al., 10 Mar 2026).

The calibration logic is multi-layered. Directional sources are intended for intra-module timing, inter-module synchronization, and water-property monitoring over tens to hundreds of meters; isotropic P-CALs are intended to illuminate large detector volumes with self-monitored, multi-wavelength light fields for geometry and optical-property fits. The 2026 paper explicitly links these systems to timing synchronization, amplitude calibration, absorption and scattering measurements, sedimentation diagnostics, and long-term drift monitoring. In P-ONE, calibration hardware therefore doubles as an environmental monitor and as an input to reconstruction systematics control (Agostini et al., 10 Mar 2026).

4. Pathfinder program and site qualification

The staged pathfinder program is the empirical foundation of P-ONE. STRAW, deployed in 2018, was built to characterize the optical properties of the Cascadia Basin water column; STRAW-b, deployed in 2020 and recovered in July 2023, extended that program to bioluminescence, LiDAR-based optical studies, DAQ integration, and deployment engineering (Bailly et al., 2021, Holzapfel et al., 2023).

Pathfinder	Configuration	Principal role
STRAW	Two 145–150 m lines; 3 POCAMs and 5 sDOM photosensors total	Optical attenuation, ambient optical background, $n$8 validation
STRAW-b	One 500 m line with 10 modules	Bioluminescence, LiDAR, PMT/SiPM timing studies, deployment and DAQ stress test

STRAW delivered the first quantitative optical site characterization. After two years of continuous operation, it measured effective attenuation lengths of $n$9 m at 365 nm, $\beta = v/c$0 m at 400 nm, $\beta = v/c$1 m at 450 nm, and $\beta = v/c$2 m at 585 nm. The 2021 design overview rounds these results to approximately 11 m, 15 m, 30 m, and 8 m, respectively, and interprets the site as absorption-dominated with scattering expected to be subdominant. STRAW also provided two years of background monitoring: single-PMT total rates had a 10th percentile of 9 kHz, a median of 38 kHz, and a 90th percentile of 548 kHz, with episodic bioluminescence spikes reaching several MHz and a clear 12.5 h modulation matching the tidal cycle. A $\beta = v/c$3-based salinity cross-check yielded $\beta = v/c$4, consistent with ONC’s independent salinity measurement of $\beta = v/c$5 (Bailly et al., 2021, Resconi et al., 2021).

STRAW-b shifted the emphasis from water attenuation to optical backgrounds and systems integration. Its standardized modules included accelerometers, magnetometers, pressure-temperature-humidity sensors, and per-channel powermeters. Its instrument suite comprised cameras and mini-spectrometers, PMT-based spectrometers, a SiPM-based Muon-Tracker, LiDAR units, and a WOM prototype. The cadence and DAQ structure were explicit: one image and one spectrum every 99.5 s for the camera/spectrometer systems, PMT time-over-threshold counts recorded at 1 kHz, daily threshold scans, daily full $\beta = v/c$6 LiDAR scans, and hourly LiDAR measurements with laser-on and laser-off conditions. Data and metadata were organized with ONC product codes and made public through Oceans 2.0/3.0, with raw digitizer outputs also synchronized to ONC servers (Holzapfel et al., 2023).

The significance of the pathfinders is cumulative. STRAW established that Cascadia Basin optical properties are compatible with large-volume Cherenkov instrumentation; STRAW-b showed that a complex, multi-instrument deep-sea array could be integrated with ONC services and operated with high uptime while generating the environmental data needed for threshold setting, trigger design, calibration schedules, and geometry planning. The pathfinder papers therefore function not merely as preliminary site reports but as direct design inputs for P-ONE (Bailly et al., 2021, Holzapfel et al., 2023).

5. Positioning, operational reliability, and long-term environmental constraints

Precise knowledge of line geometry is essential for timing-based reconstruction. The 2025 acoustic-positioning paper states that, given approximately 1 ns PMT timing resolution and the speed of light in water, P-ONE requires relative positioning resolution of about 20 cm or better so that geometry uncertainty remains sub-dominant in photon timing. Its prototype system integrates two piezo-acoustic receivers inside each pressure housing and combines them with Sonardyne cabled and autonomous beacons. In laboratory and field characterization, the behind-glass receivers reached absolute sensitivities up to $\beta = v/c$7 dB re $\beta = v/c$8 in the 10–40 kHz range. In an ocean campaign, a simple peak-finding algorithm achieved relative timing precision of approximately $\beta = v/c$9–$$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$0s at distances up to 1600 m, corresponding to range uncertainty of about 35–42 cm per receiver; with two receivers per module, the paper estimates module-center precision of about 24–30 cm per measurement, with further improvement expected from matched filtering, phase calibration, and mechanical line modeling (Collaboration et al., 17 Apr 2025).

Operationally, STRAW-b demonstrated the value of autonomous health checks and recovery logic. Its DAQ supported continuous and scheduled measurements, synchronized heterogeneous sensors, and automatically rebooted Trigger Readout Boards when needed. The collaboration also developed a Python analysis package, strawb, to synchronize files from ONC, import diverse data types, and provide basic and advanced analysis tools while accessing ONC oceanographic metadata. The pathfinder report describes the overall performance as “robust and uninterrupted,” with high uptimes for all sensors (Holzapfel et al., 2023).

The main long-term engineering constraints identified so far are not generic “deep sea” abstractions but specific failure modes. STRAW-b used a 444 m mooring line and deep-ocean-certified components, and battery-powered deployment loggers recorded mechanical stress histories intended for direct use in future design and testing. Recovery in July 2023 showed that three module failures were attributable to leaking connectors on either the mini-junction-box side or the module side, with saltwater corrosion between connectors and penetrators after water ingress. In the P-ONE context, connector selection, sealing, and corrosion mitigation are therefore first-order design priorities rather than secondary maintenance issues (Holzapfel et al., 2023).

A second constraint is orientation-dependent fouling. The 2025 long-term study of sedimentation and biofouling reports 53 months of STRAW data with 99.5% uptime and shows that upward-facing optical surfaces suffered a persistent decline in transparency after an initial quiescent phase. Fits to logistic and Gompertz growth models give critical times $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$1 yr and $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$2 yr after March 2019, equivalent to an onset of rapid losses about 2.5 years after deployment; maximum fouling rates near the inflection were $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$3 yr$$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$4 and $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$5 yr$$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$6, and the extrapolated asymptotic upward-facing transparency lay between 0% and 35% of the initial value. By contrast, a majority of downward-facing optical surfaces showed no visible fouling over five years. The paper explicitly states that P-ONE’s sensitivity to astrophysical sources will be dominated by up-going events reconstructed primarily with partially downward-facing PMTs, and it identifies several resulting design choices for P-ONE-1: no PMT faces directly upward, only the lowest module is near the most resuspension-prone STRAW heights, a subset of modules will be coated with ClearSignal, and in-situ calibration beacons remain part of the acceptance-monitoring strategy (Aghaei et al., 12 Jul 2025).

These findings correct a common simplification. Low scattering and long attenuation lengths do not by themselves guarantee stable detector performance over decadal timescales; geometry tracking, connector reliability, corrosion resistance, orientation-dependent fouling, and continuous calibration are integral to the detector concept. Conversely, the fouling results also show that the dominant astrophysical channels are not equally vulnerable across all orientations, which is why PMT orientation has become a design variable rather than a purely mechanical detail (Collaboration et al., 17 Apr 2025, Aghaei et al., 12 Jul 2025).

6. Physics reach, interdisciplinary uses, and development trajectory

The scientific reach discussed for P-ONE extends beyond generic “larger detector” arguments. In the concept papers, the detector is presented as a multi-km$$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$7 instrument for diffuse-flux measurements, steady and transient source searches, real-time multimessenger alerting, flavor studies, and high-energy cross-section constraints. Detailed P-ONE-only effective-area curves, angular resolutions, and numerical point-source or diffuse sensitivities were explicitly deferred until prototype validation, but the design emphasis on horizontal tracks and water-based timing is meant to maximize discovery potential at relevant declinations (Resconi et al., 2021).

One published forecast uses P-ONE as a dark-matter detector. In “Searching for Dark Matter Annihilation with IceCube and P-ONE,” the instrument is modeled as a large-volume, track-optimized detector for $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$8 charged-current events with effective areas exceeding IceCube’s over the relevant energy range. Under the assumptions of a NFW-like halo, a 10-year exposure, conservative IceCube-like energy smearing, modeled atmospheric and astrophysical backgrounds, and $$N_{\text{events}} = T \int d\Omega \int dE \; A_{\mathrm{eff}}(E,\Omega)\,\Phi(E,\Omega),$$9 track selection, the paper concludes that P-ONE may exceed the sensitivities of gamma-ray searches by about 1–2 orders of magnitude in the 1–10 TeV region and could push bounds beyond IceCube, potentially approaching the thermal relic abundance with a Galactic Center search and extended run-time (Desai et al., 2023).

P-ONE also has an explicit interdisciplinary dimension. The 2022 Letter of Interest invites oceanographic and marine-science participation and identifies long-term monitoring of optical bioluminescence, deep-ocean dynamics and thermodynamics, active and passive acoustics, basin-scale acoustic tomography, marine mammal and earthquake monitoring, and geothermal heat-flux studies as potential co-deployed science programs. These are not presented as replacements for the primary neutrino-astronomy mission; rather, they arise from the same distributed timing, positioning, and environmental sensing infrastructure required for the neutrino detector itself (Henningsen et al., 2022).

The development trajectory remains staged. Early concept papers described pathfinders, prototype lines, and eventual cluster-scale deployment; later subsystem papers focus more concretely on the first kilometer-scale line, its calibration hardware, acoustic system, and environmental mitigations. STRAW-b’s summary identifies the next technical steps as an extensive analysis of bioluminescence, integration with ONC current and temperature sensors, evaluation of the Muon-Tracker as a calibration tool, and LiDAR-based attenuation and scattering studies to complement STRAW. In that sense, P-ONE remains a detector under construction in the encyclopedic sense: its scientific identity is already well defined, but its final geometry, thresholds, calibration schedules, and maintenance regime continue to be determined by subsystem validation in the Cascadia Basin environment (Holzapfel et al., 2023, Agostini et al., 10 Mar 2026).