By the great wheel in the north-east corner of the Waterworks, just off the passage running northwards from the Rotting Cellar to the Deep-way, a bridge leads to a great building with a colonnaded portico. In the centre of the portico is a massive door; unlike the doors in the Rotting Cellar, this one appears "usable", suggesting that it can be opened to gain access to the building!
Alas, clicking on it, even if you are at the maximum level of 60, produces the message "Your level is too low". Now this is intriguing. Other locations or sites may at times be placed in the game with a view to activation at some point in the future, but they do not normally respond in any way at all - certainly not by appearing usable at the present time. So we may assume that this location is due to be activated in the near future; probably as part of the "back to Moria" quest thread which is supposed to be part of the forthcoming Mirkwood expansion.
However, I will pass on, will all due reservations, the following entirely unsubstantiated rumour. A kinsman who happened recently to ask a GM when this door would become accessible claims to have received the answer "At level 80". If true, this is truly astonishing. We have all been assuming that the level cap would be raised to 70 with the release of the third volume, presumably entitled Riders of Rohan, towards the end of 2009. Finding that level 80 has suddenly appeared on the event horizon is, to put it mildly, disconcerting.