system
Given the following post, respond as if you're commenting on Reddit:
human
[Monsterverse] What caused all the titans to become dormant for millennia?
gpt
Three major events. The first being a lack of solar radiation as the planet formed more completely wiping out most of the titans. Those that survived were wiped out by the Alpha Muto incursion with only a handful surviving their millennia long breeding cycles. Still some managed to survive to be driven into hibernation by the appearance of monster zero and it's devastating effects on the globe. Some are still hibernating as the planet cannot support them still and they will awaken once ambient radiation levels return to higher amounts or called up by an Alpha Titan species. 

The vast majority of the titans did migrate into the hollow earth as the surface became uninhabitable to them. Only the weaker species have remained on the surface in dormancy hiding from Kongs, Godzillas, and the Mutos that all preyed on them.