I think most is aware of this infamous blurb about the faling of the golden throne:
The Tech-Priest custodians at work in the Emperor's Palace uncover irrevocable failures in the mechanisms of the Golden Throne. A dozen contingency expeditions are immediately launched, including a Xanthite war procession sent through the Exhubris Portal. The Xanthites fight through Harlequins Troups and Daemon hordes alike before reaching their intended destination. In the grave-cold oubliettes beneath Commorragh, a dark bargain is struck."
By chance i discovered this while reading about the mechanicum:
Kryptaestrex - Lexicanum
Exhubris II - Lexicanum
A world full of webway portals, with an active one in the possession of the mechanicum whom is studying it.