User blog comment:Dumac Dwarfking/ES games and the Towers/@comment-26893431-20180626160938

''Chips of the White-Gold Tower. Ayleids who built it couldn't recreate their own Zero Stone, so they settled for Tel Var. The Daedra infesting these sewers are tap-tap-tapping away at it. Weaken it enough ... and pop, shlorp, no more Mundus.'' -Phrastus of Elinhir

Phrastus is the only in-universe source we have on the matter, and therefore, as a rule, the most reliable.