Cosmic strings (CS) are topological defects formed after spontaneous breaking of a U(1) symmetry. Remarkably, thanks to the scaling regime, CS loops constitute a long-standing source of Gravitational Waves (GW) and produce a flat GW spectrum during radiation domination, spanning many orders of magnitude in frequency.
I will discuss reasons to be excited about the possible future detection of GW from CS by the next generation of GW interferometers, in order to bring light on the existence of non-standard cosmology before Big-Bang Nucleosynthesis starts and the related Beyond-Standard-Model physics. I will discuss the possibility to probe superstring theories with moduli scale up to 10^{10} GeV, heavy dark photon with kinetic mixing as low as 10^{-18}, heavy ALPs with masses between 1 GeV and 10^{10} GeV or PBHs below 10^9 grams.