The results of large-scale experiments on turbulent flame propagation and transition to detonation in a confined volume of lean hydrogen-air mixtures are presented. The experiments were in a strong concrete enclosure of 480 m(3), and of 69.9 m length. The experimental volume consists first of a channel (34.6 m length, 2.3 m height, 2.5 m width) with or without obstacles, a canyon (10.55*6.3*2.5 m), and a final channel. Ignition was with a weak electric spark at the beginning of the first channel. The effect of hydrogen concentration (9.8%-14% vol.) on turbulent flame propagation and transition to detonation was studied. The obstacle configuration in the first channel (blockage ratio 0.3, 0.6, and no obstacles), exit cross section to the canyon (1.4, 2, and 5.6 m(2)), and vent area at the end (0, 2.5, and 4 m(2)) were varied in the tests. Details of turbulent flame propagation, of pressure field, and of detonation onset are presented. A minimum of 12.5% of hydrogen was found to be necessary for transition to detonation. This is a much less sensitive mixture than those in which the onset of spontaneous detonation has previously been observed (minimum of 15% of hydrogen in air). The effect of scale on the onset conditions for spontaneous detonation is discussed. The characteristic geometrical size of the mixture for transition to detonation is shown to be strongly related to the mixture sensitivity.