The correct answer is bank vaults as stated in Alan Weisman's book "The world without us" when he tries to depict the New York city scenery long after humans have disappeared from the earth. The reason why this would most likely happen is that quite a few of this Bank Vaults were made out of steel reinforced concrete, making them technically indestructible, unlike, for example, museum vaults, which are built to preserve with climate control rather than to protect, therefore using softer materials.
