This may have been pointed out before, but not where Google and I could find it.
Harry, speculating in Chapter 25:
Some intelligent engineer, then, had created the Source of Magic, and told it to pay attention to a particular DNA marker.
The obvious next thought was that this had something to do with "Atlantis".
Harry had asked Hermione about that earlier - on the train to Hogwarts, after hearing Draco say it - and so far as she knew, nothing more was known than the word itself.
It might have been pure legend. But it was also plausible enough that a civilization of magic-users, especially one from before the Interdict of Merlin, would have managed to blow itself up.
The line of reasoning continued: Atlantis had been an isolated civilization that had somehow brought into being the Source of Magic, and told it to serve only people with the Atlantean genetic marker, the blood of Atlantis.
"Worse than any peril I know," said Albus. "But probably not worse than whatever erased Atlantis from Time."
The Friendly AI Critical Failure Table:
34: The programmers and anyone else capable of explaining subsequent events are sent into temporal stasis, or a vantage point from which they can observe but not intervene. The rest of the world remains as before, except that psychic powers, ritual magic, alchemy, et cetera, begin to operate. All role-playing gamers gain special abilities corresponding to those of their favorite character.
It's a reference to Harry Potter and the Wastelands of Time. Ancient magic, unlimited timetravel, and correspondingly more powerful opponents.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: