The Underwater city is an ancient city, situated on the bottom of one of Gielinor's many oceans. It was mentioned by Guthix in Issue 24 of the God letters. The city was submerged at an unknown date and eventually forgotten. Some consider it to be no more than a myth. However, Guthix also mentions that its inhabitants are making efforts to re-establish their contact with the rest of the world.
Many players thought that this area was the place that could be seen from Commander Zilyana's chamber in the God Wars Dungeon. However, Jagex later confirmed that this was an abandoned area that was originally intended for an underwater passage to Zamorak's Fortress which was replaced by the current swim across the river due to time constraints.
A quote from Guthix in the God Letters:
"If thou art asking of the city beneath the waves, it hath been submerged for many centuries, and most commonfolk consider it naught but a myth. I tell thee true, however, that such a place doth indeed exist, and even as I write this there are those working to re-establish access between this place and the outside world that hath all but forgotten it over these past centuries."