When Christ was crucified and He died on the cross the world mourned and there were severe tempests, and massive natural disasters. We read in the Book of Mormon about cities collapsing, some being completely buried, covering up the sins of its inhabitants, others being swallowed up by the sea. If the Lord did not want to leave these cities standing in view, I believe He is entirely capable of concealing them from His eyes and ours. That said, there are still signs of civilizations descended from the people of the Book of Mormon, but it seems the proof that they are connected is debatable. In my mind it is an issue of faith and that is the way the Lord will have it. Think about it. As it is the world is struggling with Christianity even with proof of biblical cities and artifacts still in existence! But that is not enough for many to believe that Christ walked the earth and if there are people that believe He did, well then, maybe He was just a great man. Had Jerusalem been destroyed like some cities of the Book of Mormon were and covered up forever I can imagine the unbelief of the world population today. But we need that anchor, that evidence that Christ did walk the Earth. Without that, perhaps the less valiant of the world would fall into Satan's grip given that they, in their ignorant state, would be left to their own devices. If people have a hard enough time believing that Christ lived and that there is a God, then what difference would it really make if the city of Zarahemla really was found or if perhaps a gold plate or two had been discovered. There would still be something to criticize, something to doubt, something to dispute and something more to disbelieve to people's condemnation. Just my two cents.