Between about 1964 and 1973, large amounts of older material stored in the BBC's various video tape and film libraries were either destroyed,[note 3] wiped, or suffered from poor storage which led to severe deterioration from broadcast quality. This included many old episodes of Doctor Who, mostly stories featuring the first two Doctors: William Hartnell and Patrick Troughton. In all, 97 of 253 episodes produced during the first six years of the programme are not held in the BBC's archives (most notably seasons 3, 4, & 5, from which 79 episodes are missing). In 1972, almost all episodes then made were known to exist at the BBC, while by 1978 the practice of wiping tapes and destroying "spare" film copies had been brought to a stop.
Who were the first two actors that played Doctor Who? How many of the episodes produced in the first six seasons are not in BBC's archives? Which seasons is the BBC missing a total of 79 episodes? When did the practice of wiping tapes stop? What years saw the most loss of old shows in the BBC archives?