  Accurately reproducing the thin cold gas discs observed in nearby spiral
galaxies has been a long standing issue in cosmological simulations. Here, we
present measurements of the radially resolved HI scale height in 22
non-interacting Milky Way-mass galaxies from the FIREbox cosmological volume.
We measure the HI scale heights using five different approaches commonly used
in the literature: fitting the vertical volume density distribution with a
Gaussian, the distance between maximum and half-maximum of the vertical volume
density distribution, a semi-empirical description using the velocity
dispersion and the galactic gravitational potential, the analytic assumption of
hydrostatic equilibrium, and the distance from the midplane which encloses
$\gtrsim$60 per cent of the HI mass. We find median HI scale heights, measured
using the vertical volume distribution, that range from ~100 pc in the galactic
centres to ~800 pc in the outskirts and are in excellent agreement with recent
observational results. We speculate that the presence of a realistic multiphase
interstellar medium, including cold gas, and realistic stellar feedback are the
drivers behind the realistic HI scale heights.
