The Unending well is a bottomless chasm that tears through the Eastern face of Mount Silvertine. An extensive number of bridges had been built to cross it, most notably the Bridge of Khazad-dum, but it is unkown whether many of these remain to this day.
The chasm is said to lead so deep that it ends on the top of Mount Silvertine. This information was inferred from the account of the death of the Balrog, where a fall down the well lead the combatant to eventually fall onto Durin’s tower.
The faces of the chasm therefore is incident to most levels and deeps of Khazad-dum.