- Start
- Jul 31, 202590% CONFIDENCEfrom the source
Hyper-Kamiokande Neutrino Observatory Cavern Completed
- The University of Tokyo completed excavation of the Hyper-Kamiokande cavern on July 31, 2025, announced on 5 August 2025; the water Cherenkov detector is under construction about 600 m underground in Hida City, Gifu, Japan[2]
- The cavern is a dome about 69 m in diameter and 21 m high above a cylinder 73 m high, with about 330,000 cubic metres of rock removed; the dome and cylinder heights together match the pin source's correction that "When complete, the cavern will be 94m high"[1][2]
- Excavation finished about six months behind the original schedule: dome excavation began November 2022 and the cylinder in October 2023, using a pre-dug 3.4 m vertical shaft[2]
- The two links differ on when observations begin: the newer release has operation "scheduled to begin 2028", where the March 2024 paper said operation was "expected to start in 2027" - the newer date follows the six-month excavation slip[2][3]
- Next steps from August 2025: converting the cavern into a tank of 260,000 cubic metres of ultra-pure water, a two-layer detector structure starting in 2026, all in-tank components installed by 2027, while KEK upgrades the J-PARC beam and builds a new intermediate detector in Tokai[2]
- The detector itself is given as 71 m high and 68 m in diameter holding 0.260 million metric tons of ultra-pure water, a fiducial mass of 0.188 million tons - an order of magnitude more than Super-Kamiokande, or 8.4 times its fiducial volume - with over 20,000 new photodetectors[2][3]
- The collaboration is led by the University of Tokyo and KEK; it counted about 560 researchers from 101 institutes in 21 countries in March 2024 and about 630 researchers from 22 countries by July 2025[2][3]
- Its goals are to measure neutrino properties and search for proton decay: leptonic CP violation covering 60% of parameter space at 5 sigma or better after 10 years (over 8 sigma if delta CP = -pi/2), proton decay p -> e+ pi0 potentially beyond 1e35 years, neutrino mass ordering, and solar and supernova neutrinos[2][3]
- The project's start is dated slightly differently: the release says the project began in February 2020, while the paper dates the budget approval by the Japanese Diet to January 2020[2][3]
- Earlier excavation steps: the yard was prepared from 6 May 2021 (~10,000 m2), a 2.0 km access tunnel (1979.5 m, 5 m x 5 m) was done in 9 months and completed February 2022, approach tunnels of 385 m, 413 m and 154 m ran March-August 2022, a 445 m circular tunnel with sub-cavities took about 4 months, and a water purification cavity of 101 x 17 x 16 m was completed May 2023[3]
- The pin's video source, The B1M's "Why Japan is Hollowing Out a Mountain" (published 2024-02-28), predates completion and gives no date or cost, but adds that the observatory sits 600 m below the mountain peak and that the tank water is treated with ion exchange resin and other means "to create water so pure that the transmitted light does not attenuate for more than 100 metres"[1]
References 389% CONFIDENCE
The first entry is always the pin's source. Overall confidence is a weighted average of how firmly each reference supports the start and end times used above; a reference counts half as much for every 180 days older than the newest.
- [1]90%youtube.com/watch?v=aSYL0MHMwogyoutube.com· Posted Sep 15, 2026· Starts Jul 31, 2025 ✓· 78% of score
Happened as dated, per the source: "the University of Tokyo completed excavation of the colossal cavern that will house the main detector volume of Hyper-Kamiokande" on July 31, 2025.
- [2]88%interactions.org: Excavation of the Colossal Cavern for Hyper-Kamiokande Completedinteractions.org· Published Jul 31, 2025· 16% of score
The press release confirms University of Tokyo crews completed excavating the Hyper-Kamiokande cavern - roughly 330,000 cubic meters of rock - "on July 31, 2025," matching the pin's stored date exactly, about six months behind original schedule.[1]
- [3]80%Frontiers in Physics: Hyper-Kamiokande construction status and prospectsfrontiersin.org· Published Nov 1, 2024· 6% of score
The page could not be fetched, but this Frontiers in Physics paper on Hyper-Kamiokande's construction status, published November 2024, predates the actual cavern completion described by the pin and offers only earlier-stage academic context rather than confirming the July 2025 date itself.
Suggest a correction
Something missing or wrong? Say it in your own words: a link that backs this pin up, a different start or end date and why, or a fact it lacks or gets wrong. The AI checks it against this pin's sources, searches for better ones, and adds any page that backs you up. The pin's own sources still count most.
