Gotta be under a city for maximum carnage when it awakens!
Plot twist. It's laid a couple hundred eggs while sleeping. It's awakening will be preceded by dozens of CR 20-ish mini-spawn...
No, no Tarrasque breeding grounds please. I've already got enough PTSD.
Besides, we should absolutely save the Tarrasque for PF3e. If the Whispering Tyrant coming back was enough to set off a new edition, imagine the Tarrasque coming back and wiping a few nations off the map.