The largest known experiment on hydrogen-air deflagration in open atmosphere has been analysed bymeans of the large eddy simulation (LES) technique. The LES model of premixed combustiondeveloped at the University of Ulster was applied to investigate the underlying physics of thephenomenon. The LES combustion model is based on the progress variable equation to simulate thepropagation of the premixed flame front. The gradient method is applied for a source term in theprogress variable equation to decouple the physically grounded turbulent burning velocity from anumerical grid. The transition from laminar to turbulent combustion has been considered through twomain physical phenomena, i.e. hydrodynamic flame front instability and turbulence generated by theturbulent flame front itself. The hydrodynamic instability of the premixed flame front has beenpartially resolved by LES and its SGS effect has been modelled by the Yakhot?s model of premixedturbulent combustion. Contrary to the hydrodynamic instability the turbulence generated by flamefront itself takes place at SGS level only. It has been modelled based on the maximum value for flamefront self-induced turbulence predicted by Karlovits et al. in 1951 and the transitional distancedetermined by Gostintsev et al. The chemistry and effects of the selective diffusion at scales of realflamelets are taken into account through the value of laminar burning velocity 1.91 m/s forstoichiometric hydrogen-air mixture. The LES model has been successfully validated againstexperimental data on the dynamics of flame propagation from the ignition source to the radius of 20m, flame shape, positive and negative phases of the pressure wave generated by the explosion atdistances up to 80 m. The model is built from the first principles and no adjustable parameters havebeen applied to get very good agreement with the experiment.