A fast radio burst is a flash of radio waves lasting only a few thousandths of a second yet releasing as much energy as the Sun does in days. First found in 2007, thousands are now catalogued, some repeating and most one-off. Their dispersion as they cross intergalactic space makes them cosmic distance probes. A burst from a magnetar inside our own galaxy in 2020 gave the strongest evidence that these extreme neutron stars are a major source.