  HI and CO observations indicate that the cold gas in galaxies is very
turbulent. However, the turbulent energy is expected to be quickly dissipated,
implying that some energy source is needed to explain the observations. The
nature of such turbulence was long unclear, as even the main candidate,
supernova (SN) feedback, seemed insufficient. Other mechanisms have been
proposed, but without reaching a general consensus. The key novelty of our work
is considering that the gas disc thickness and flaring increase the dissipation
timescale of turbulence, thus reducing the energy injection rate required to
sustain it. In excellent agreement with the theoretical expectations, we found
that the fraction of the SN energy (a.k.a. SN coupling efficiency) needed to
maintain the cold gas turbulence is $\sim 1$%, solving a long-standing
conundrum.
