The official contaminated zones saw a massive clean-up effort lasting seven months. The official reason for such early (and dangerous) decontamination efforts, rather than allowing time for natural decay, was that the land must be repopulated and brought back into cultivation. Indeed, within fifteen months 75% of the land was under cultivation, even though only a third of the evacuated villages were resettled. Defence forces must have done much of the work. Yet this land was of marginal agricultural value. According to historian David Marples, the administration had a psychological purpose for the clean-up: they wished to forestall panic regarding nuclear energy, and even to restart the Chernobyl power station.