From the Comprehensive Rules:
If a creature returned to play with unearth would leave the battlefield for any reason, it's exiled instead — unless the spell or ability that's causing the creature to leave the battlefield is actually trying to exile it. In that case, it succeeds at exiling it. If it later returns the creature card to the battlefield (as Oblivion Ring or Flickerwisp might, for example), the creature card will return as a new object with no relation to its previous existence. The unearth effect will no longer apply to it.