Well, this is interesting!

Every year, the British Film Institute holds a “Lost and Presumed Wiped” event. It highlights the fact that, in the 1970s, the BBC exercised a  policy of recording over their own archives after a certain period of time—they would lose the right to air them, or they would assume no one would want to rewatch them, or they would just need extra tapes, but for whatever reason, the Powers That Be figured no one would want to watch decade old episodes of Doctor Who, and they would wipe the originals.

Of the 253 episodes of Doctor Who broadcast between 1963 and 1969, 108 are missing from the archives and thought to have been wiped. (I just linked to Doctor Who's lost episode Wikipedia article, but the entire parent article about the practice of archive wiping worldwide is pretty interesting, if that's your sort of thing.) Patrick Troughton's run as the Second Doctor was hit the hardest by the wipings, and only low quality, silent clips of the Doctor's first regeneration (changing the character from William Hartwell's First Doctor to Troughton's Second) have been located at this time. (It should be noted that audio recordings of all the lost episodes exists, but only thanks to the enthusiasm of the show's geeky fans at the time.)

So! 108 episodes lost, with only two full episodes located in the time since the '90s. And then, Who fans woke up this week and there were two more.

The first found was "Airlock", the third episode of "Galaxy 4", an up-until-now entirely lost First Doctor four-parter following a war between the reptilian Rill and the sexy Drahvins (New Who fans may remember these beezies helping to lock Matt Smith's Eleventh Doctor in the Pandorica in Series 5).

The second is episode two of the "Underwater Menace" four-parter. The rediscovered episode, in which the Doctor tries to stop a mad scientist from raising the city of Atlantis, is now the earliest surviving Second Doctor appearance and the first extant appearance of companion Jamie McCrimmon (who appeared in more episodes than any other companion); only episode three of that four-parter still exists.

Radio Times has a great piece about the actual discovery. The reels are a little chewed up, but the archivists at BBC are working hard to make up for lost time and 20/20 hindsight. Two down, 106 to go.