An international collaboration of European astronomers, together with Indian and Japanese colleagues, have seen evidence for ultra-low-frequency gravitational waves, which is expected to originate from pairs of supermassive black holes found in the centres of merging galaxies. This is the result of more than 25 years of observations with the most sensitive radio telescopes in Europe and India, including the Westerbork Synthesis Radio Telescope (WSRT). In doing so, they have opened a new window on gravitational wave research. These gravitational waves contain information about the Universe's best-kept secrets. The research has been published in a series of articles in the professional journal Astronomy & Astrophysics.