In a vast, dark hall, they're searching for a single person who makes almost no sound. That's how physicists hunt for sterile neutrinos — ghostly particles that pass through matter, leaving barely a trace.
Inside every atom lives a proton. For a long time, it was considered as eternal as matter itself. But theories that unify all forces of nature predict that, sooner or later, the proton will decay. The only question is how long we have to wait.
In the Japanese detector Super-Kamiokande — a giant tank of ultra-pure water — physicists searched for signs of such a decay.