\section*{Design of P-ONE}
An era of mutli-cubic-kilometre neutrino telescopes is now technologically possible thanks to the experience gained over the last decade in the development of deep ocean infrastructure, for example by ONC, and in the construction of neutrino telescopes by IceCube\cite{Aartsen:2016nxy}, ANTARES\cite{Aguilar:2006rm} and Baikal-GVD\cite{Avrorin:2019dli}.  With the challenge of a stable system within which the construction and operation of deep ocean neutrino telescopes is largely addressed, the next hurdle is the physical structure of the  telescope itself and the associated costs. With a characteristic deep ocean (lake) water optical attenuation length of 20-70\,m, a uniform infill array, similar to IceCube\cite{Aartsen_2017}, would require thousands of instrumented lines distributed
over the entire volume. This poses a daunting challenge that,  following from the pioneering effort of the DUMAND project\cite{Babson:1989yy}, can be overcome by introducing a segmented neutrino telescope design (see Fig.\,\ref{fig:Fig2}).
For the search of individual astrophysical sources, P-ONE will primarily target horizontal high energy muon tracks, but will also be sensitive to high energy induced showers. 
 The muon produced in the interaction of a very high-energy neutrino can travel several kilometers in water or ice. The Cherenkov light produced by the propagating muon provides enough information for the reconstruction of the incoming neutrinos with an angular resolution of a fraction of a degree\cite{Aartsen_2017}. 
A segmented array sampling approach would retain sufficient information to reconstruct the incoming direction and the energy of the track while significantly reducing the number of mooring lines required to cover the large volumes  (see Fig.\,\ref{fig:Fig2}). 
The P-ONE {\it Explorer}, corresponding to the first 10-string segment, has a planned deployment in 2023-2024 during a marine operation scheduled to last four weeks. A total of 20 photo-sensors and at least two calibration modules will compose each individual string of the Explorer. The remainder of the telescope array, comprising on order 70 strings, are planned for deployment between 2028-2030. 
As the pathfinder stage for the complete telescope, the Explorer will not only pave the way for a successful P-ONE installation  but also improve on existing searches for $10-100$\,TeV energetic $\nu_\tau$ of astrophysical origin\cite{Aartsen:2020aqd}, probe the production of long-lived exotic particles in the atmosphere, and complete current studies on the systematic uncertainties of cosmic ray interaction patterns. 

\begin{figure}[h!]
\centering
\includegraphics[width=1.0\textwidth]{Fig2.png}
\caption{(left) Design of the Pacific Ocean Neutrino Experiment, as presented in this work, consisting of seven sectors of 10 strings optimized for energies 10\,TeV-10\,PeV. The light pattern produced by a 50\,TeV neutrino is included shown within the detector. (Right) Neutrino and anti-neutrino effective areas for charged current interactions in P-ONE. The three different lines represent the effective area in different declination bands. The dotted line at 50\,TeV shows the equivalent energy of the event view in the detector. The effective area is calculated by requiring a minimum deposit of 100\,GeV of energy, and a minimum track of 100\,m in one sector.  The effective area calculations also take into account the absorption effect, which is more relevant for high-energy neutrinos transiting deep through the Earth\cite{Aartsen:2017kpd, Bustamante:2017xuy}. Neutrinos from the horizon are nearly absorption-free, even at energies above the PeV scale.}
\label{fig:Fig2}
\end{figure}

P-ONE will be installed and operated within one of the world's largest and most advanced cabled ocean observatories, ONC. ONC is composed of many infrastructures, the largest being the North East Pacific Time-series Underwater Networked Experiment (NEPTUNE). 
The NEPTUNE cabled ocean observatory\cite{Neptune:2010},
completed in 2009,  comprises an 800\,km loop of telecommunication fibre-optic cables to power and transfer data to a variety of sensors connected to five nodes across the Juan de Fuca tectonic plate, approximately 200\,km off the coast of British Columbia, Canada.
The installation of the scientific instrumentation is effectively {\it plug and play}, allowing for highly modular deployment and maintenance. 

The node deployed in 2009 at Cascadia Basin at a depth of 2660\,m has been selected to host the P-ONE detector array. The wide and flat sediment surface provides ideal conditions to host a neutrino observatory, namely a large abyssal plain with temperature below 2\,$^o$C.  The Cascadia Basin is home to an assortment of well-adapted organisms, some of which emit bioluminescence light that is a source of background for a neutrino telescope. 
To fully qualify the Cascadia Basin as a site to host a multi-cubic-kilometer-scale neutrino telescope, a two-line calibration setup - the STRings for Absorption length in Water (STRAW)\cite{Bedard:2018zml} - was deployed in Spring 2018 and connected to the existing NEPTUNE node. STRAW has been continuously monitoring the deep Pacific Ocean conditions: without the need of any repair or maintenance, STRAW works in near real-time with a duty cycle of 98\% since completion of the commissioning phase in January 2019. 
STRAW has provided a first {\it in-situ} measure of the effective attenuation length -- at a wavelength of 465\,nm -- of 35\,$\pm$\,5\,m as well as the monitoring of the local bioluminescence activity over a period of more than 12 months. 
 The photon rate observed in the 400-600 nm wavelength window shows periodicity of 12 and 24 hours connected to the semi-diurnal and diurnal tides, respectively. The level of irradiance observed in the deep ocean is in the range of 10$^{-11}-10^{-9}$\,W/m$^2$, and hence quite low. No periods of significant increased environmental background were observed yet.
STRAW is scheduled to continue monitoring of the site through 2021.
 A second investigative mission, denoted STRAW-b, is presently under construction with deployment planned in 2020.  The STRAW-b advanced optical monitoring will continue the qualification of the site while, in parallel, the P-ONE collaboration finalizes the design of the {\it Explorer} detector array.

\section*{A distributed planetary network of telescopes}
From the first indicative high energy neutrino associations with blazars\cite{Giommi:2020hbx},
it is evident that neutrino astronomy finds itself at the tipping point. A global effort has been mobilized\footnote{\url{http://www.globalneutrinonetwork.org}} in order to fully explore the sky at the highest energies and to reveal the most powerful cosmic accelerators in the Universe. 
Contributing parties to the effort are the neutrino telescope ANTARES (see \url{https://antares.in2p3.fr}, in operation since 2008,  KM3NeT (see \url{https://www.km3net.org}), which is under construction in Italy and France, the Baikal-GVD (see \url{https://baikalgvd.jinr.ru}) detector under construction in Russia (Lake Baikal), 
IceCube and IceCube-Gen2 at the South Pole, and in the near future P-ONE. When combined to behave as a single distributed planetary instrument, the acceptance for astrophysical neutrinos at above the PeV-scale covers the entire sky. In fact, as recently verified\cite{Aartsen:2017kpd, Bustamante:2017xuy}, the cross section for the neutrino--nucleon interaction increases with increasing neutrino energy, making the Earth significantly less transparent to neutrinos with energies above 50\,TeV; less than 20\% (5\%) of the neutrinos with energy of 100 TeV (1\,PeV) can cross the Earth at vertical zenith angle cos($\theta$)\,=\,-0.8., see also Fig.\,\ref{fig:Fig2}. Neutrino telescopes are therefore effectively blind to very high energy neutrinos crossing the Earth.
In order to obtain peak all-sky neutrino exposure, it will be essential to combine the information from the various telescopes.
Each telescope is designed to send alerts to  the entire astronomy community, including the other neutrino telescopes, making it possible  to follow in real-time the temporal evolution of a transient event emitting in the PeV energy region (e.g. GRBs).
Therefore, while the P-ONE telescope is  designed to operate individually, providing partial sky coverage, the real impact for the exploration of the non-thermal universe via neutrinos is anticipated through leveraging the strengths of the global suite of telescopes as a unified instrument~\cite{resconi_elisa_2019_3520454}.
To quantify an order of magnitude gain from the combination of neutrino telescopes distributed around the Earth, we present here the neutrino acceptance defined as $\int_{0}^{\infty} A_{eff}(\delta, E_\nu) \cdot E_\nu^{-\gamma}$\,dE$_\nu$, where $A_{eff}$ is the neutrino effective area and $E_\nu$ the energy of the neutrinos. The gain in acceptance is sensitive to the intrinsic spectrum of possible neutrino sources; for comparison we therefore fix the neutrino spectral signal to $E_\nu^{-2.5}$ ($E_\nu^{-2.0}$).  In the scenario where four IceCube-equivalent telescopes are in operation around the globe (Lake Baikal, Mediterranean Sea, Pacific Ocean, and South Pole) an effective gain of more than a factor 100 (20) with respect to the single IceCube operation would be obtained. 
 The main improvement is observed in the Southern hemisphere, where IceCube data are dominated by atmospheric muons. Even including the atmospheric muon and neutrino veto capabilities\cite{Schonert:2008is}, the corresponding regions of the sky affected by this background results in a minimal effective area for neutrinos. Additional neutrino telescopes covering these regions with acceptance near the horizon will produce a significant relative increase in sensitivity. 


In the search for persistent sources, the telescopes' data may be combined offline. If also sychronized in time, the network of neutrino telescopes provides a nearly all-sky exposure to high energy transient sources to the multimessenger community (e.g. Open Universe, \url{http://www.openuniverse.asi.it}), as well as the prompt follow up of alerts issued by gravitational wave detectors and gamma-ray facilities.  
An all-sky acceptance for neutrinos from the GeV to the PeV-scales in connection to the whole of multimessenger astronomy creates ideal conditions to discover the most extreme particle accelerators in the cosmos — the sources of the ultra high energy cosmic rays - and study the deepest mysteries of the non-thermal Universe.



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