Another interesting point.... Biblical Scholars say that John did not write his gospel until at least the 2d Century AD, well after the destruction of Jerusalem, and that it was not written by the Desciple John, but by someone who assumed his name.
If so, then how did this individual find out about the pool, I wonder?
If that part of the gospel was actually written in the 2nd Century, I would assume that the knowledge of the pool itself was still there 200 years later - it didnt become forgotten until later.
The oral tradition was pretty strong back then - its quite possible that the Gospel of John was handed down in an oral tradition and not formally recorded until some generations had passed. Locals of the 2nd Century would probably have a very good idea of how Jerusalem was laid out in the time of Christ, but some centuries later, after the various barbarian and Islamic invasions, the Crusades and however many temporary evacuations and resettlements of Jerusalem, local knowledge probably lost a lot of old geographic reference points.