The BBC Archives are a collection of materials about and from the history of the British Broadcasting Corporation.
Not all the episodes of the First and/or Second Doctor eras were preserved in the BBC Archives, as many were junked during the 1970s as being of no further marketable value — for example, only episode two of Season 5's six-part story The Abominable Snowmen exists there.
Fortunately, over the years, many previously missing 1960s black and white Doctor Who episodes have been found and returned to the BBC Archives by private film collectors or overseas television stations.
All the missing 1960s Doctor Who episodes exist in soundtrack form, thanks to fans who recorded the episodes off-air on reel-to-reel tape recorders from the original transmissions; while off-screen tele-snaps taken by John Cura of several stories not only show what the episodes looked like but also are the only existing visual representation.