Just after 2 a.m. on 4 January 2017, the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) detector located in Hanford, Washington, registered a tiny ripple in the fabric of space-time. Three milliseconds later, its twin detector, located some 3,000 kilometres away in Livingston, Louisiana, picked up an identical oscillation. Each ripple comprised a stretching and squeezing of space by less than 1 part in 1 billion trillion (1021), and together they constituted the discovery of the gravitational-wave signal GW170104. The details of this observation are reported by Abbott et al. (the LIGO Scientific Collaboration and the Virgo Collaboration)1 in Physical Review Letters, adding to a growing set of data that will help astrophysicists to discover the life stories of massive stars.