Played our first game a few nights ago. I was the Cave player and I played 'Quake'. This was before the Collapse so I assumed that the removed tiles just went back to my stack o' tiles but now I'm thinking that was totally wrong and made the event card pointless?
We must move forward, not backward, upward, not forward, and always twirling, twirling, twirling towards freedom!
It works just like the Dragon's Wrath power. If it's before the collapse, all non-Crystal tiles go back under the deck. If it's during the collapse, all of the tiles remain out. Crystal tiles remain out and count toward the Cave's victory condition no matter when they're removed.
If the map was split you push the pieces back together, then fill open edges. Sometimes you do end up replacing the same number of new tiles as those that were removed.