From 1963 to 1969, the BBC erased about 120 Doctor Who episodes. There are several explanations for this--they needed space in the archives, they had legal problems involving transmission, they wanted to save money be re-using old tapes, or the recordings had deteriorated because of bad storage practices, but this remains about the dumbest move of all time by a TV network--unless you think cancelling Star Trek takes that particular cake--because the VCR had already been invented and was becoming popular.
There followed decades of trouble and expense--still going on--in a worldwide search for the missing material in basements and storage rooms of TV studios, and a desperate call for Doctor Who fans to send in what they had recorded illegally off their TV screens. Much was recovered and heroically re-mastered--found in such places as the basement of a Mormon church, local TV stations in Nigeria and Hong Kong, and the Australian Government Censor Office, where particularly horrific scenes were snipped out to protect the delicate sensibilities of Australians and, for some reason, kept.
However, a great deal of the First and Second Doctors' material was lost forever. For example, the earliest stories alternated between science-fiction and history, and many of the latter were the best. The BBC was much more at home with Shakespearean rhetoric and historical sets and costumes than it was with aliens and monsters, and the historical tales had a much smaller silliness quotient. Of these, The Aztecs, The Romans, and The Gunfighters survived intact, but the Reign of Terror is available only with animated restoration of the 4th and 5th episodes, The Crusade is known only by a few fragments, and the apparently brilliant Marco Polo is entirely lost.
Marco Polo is a seven-part serial in which The Doctor and his companions meet Marco Polo and Kublai Khan. The TARDIS is damaged landing in the Pamir Mountains of the Himalayan Range in 1289 and the travellers are picked up by Marco Polo's caravan, then taken via the Silk Road across the bandit-ridden Gobi Desert to appear before the Emperor Kublai Khan. It was well-received by viewers and well-reviewed by critics--in fact it is widely considered one of the best First Doctor stories--but in 1967 it was erased, and no copies exist. There are audio tapes and still shots, including some in full colour showing spectacular costumes and sets that were only seen on screen in black and white, and one striking picture of the TARDIS lashed to an oxcart as a gift for Kublai Khan.