During the conflict, we discover that it was our character that is in fact responsible for the destruction of ancient Saarthal! We were able to protect the linearity of time by burying the Eye below the smoking ruins of the human city.
The destruction of Saarthal would be an intended solution or a tragic accident, depending on the faction the player sides with.
To travel back in time, we might discover an Egg laid by Alduin. We try to make an omellete, but by breaking the egg, a rift in time pulls us into the past.
EDIT: Or maybe it is the return of a certain vampire snake-race across the ocean. The Akiviri soldiers come to help fight Alduin but discover they are too late, and instead joing the Empire once again to defeat the Aldmeri Dominion.