The term canyon is actually a misnomer, as the the long forked crack in the planet's crust cutting through Agog City's Xenon District is actually a variety of bottomless pit. This has long been used to dispose of garbage and wastes that even The Vat Guild would not deign to accept in trade.
In the thousands of years of The Controller's reign, he has seen fit to disallow expansion of the city walls or construction within several miles of the city. The poorer inhabitants of the Xenon District have long been encouraged to build dwellings anchored to the walls of the bottomless canyon and it's sides are encrusted with a wide variety of constructions and has become known as The Tenement Canyon.
Due to the unregulated and cheap methods used to build them, is is not uncommon for portions of this construction to come loose and plunge into the abyss. As well, this bottomless gulf intersects with the lightless gulfs deep in the planet's crust and it's inhabitants often slither or flap their up to the tenements to prey upon the inhabitants. The more callous of Agog City's oligarchy consider such misfortunes to be side benefits of confining the poorest classes to the canyon.