In the search to understand the cosmos, neutrinos—subatomic particles created in nuclear reactions—have become critical clues to some of physics’ most complex questions. Produced in vast quantities by processes such as nuclear fusion in the Sun, neutrinos are hard to capture due to their weak interactions with matter. On Earth, advanced detectors have been built to study them, including Japan’s Kamiokande and the IceCube Neutrino Observatory in Antarctica. Now, astronomers are setting their sights on a new frontier for neutrino observation: the depths of the Pacific Ocean.

Neutrinos, second only to dark matter in their mystery, play a key role in nuclear reactions. For example, every second, around 60 billion neutrinos produced in the Sun’s core pass through a thumbnail-sized area on Earth. Detecting neutrinos is challenging, however, due to their minimal interactions with other particles. Most pass through matter without leaving a trace. Yet when neutrinos do collide with water molecules, they emit a tiny flash of light. Special photoreceptors in detectors like Kamiokande, which used over 50,000 tons of water, or IceCube, which harnesses an entire cubic kilometer of Antarctic ice, capture these brief flashes, allowing researchers to measure neutrino direction and energy.

Despite these advances, ultra-high-energy neutrinos remain rare and difficult to detect. IceCube, after ten years of observation, has managed to capture only a handful of these high-energy particles, suggesting a larger-scale solution is needed. High-energy neutrinos are highly significant because they are produced by extreme cosmic events such as supernovae or colliding black holes, making them prime targets for research.

The proposed Pacific Ocean Neutrino Experiment (P-ONE) aims to transform a large area of the Pacific Ocean into a neutrino detector, expanding our ability to capture these elusive particles. The concept is straightforward: using long strands of photodetectors, approximately a kilometer in length, placed over a mile beneath the ocean surface, P-ONE would act as a “natural” neutrino detector. Designed as seven clusters, each consisting of ten strings with 20 optical elements, P-ONE would hold a total of 1,400 photodetectors spread across several miles of the Pacific, far exceeding the coverage of IceCube.

However, the unique properties of the ocean pose new challenges. Unlike the controlled ice of Antarctica, the ocean is a dynamic and complex environment, with the detectors needing to account for salt, plankton, and other particles that interfere with light, as well as constant movement in ocean currents. This variability demands continuous calibration to maintain measurement accuracy. To address these issues, P-ONE’s developers have planned an initial two-strand demonstration phase to test the concept before scaling up to full capacity.

If successful, P-ONE could enable new insights into the universe by capturing more of the rare, high-energy neutrinos and tracing their origins. Ultimately, the vast and deep waters of the Pacific could become the next frontier in astrophysics, helping researchers explore the most energetic and distant events in the cosmos.