Conceptual illustration of a sequence of black hole mergers that could have lead to the gravitational wave signal GW190521 detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) gravitational wave detector laboratories in the USA, and the Virgo interferometer in Italy, on 21st May 2019. GW190521 showed the merger of two black holes with masses 85 and 66 times that of our sun (solar masses), resulting in a black hole with a mass of 142 solar masses. This is the largest merger observed to date. Theoretical models suggest that black holes between 65 and 120 solar masses cannot form from a collapsing star and so it is thought that the two black holes that merged were themselves the results of previous mergers.