The evolution of primordial fluctuations in the early universe is accompanied by a production of gravitational waves. The so-called induced gravitational waves are the cosmic messenger of primordial fluctuations on the smallest scales, which may have been generated during cosmic inflation. In this talk, I will provide a general overview of the induced gravitational waves and discuss how we can use them to probe the primordial universe. At the end, I will focus on signals from the primordial black hole dominated early universe. The topic of this talk is one of the research themes of the new Emmy Noether group at the ITP Hannover.