Mirage Tower (げんえいのとうPhantom Tower) is a spooky tower that is only in Pokémon Emerald. It is on Route 111 and disappears and reappears at random times. The tower has four floors and the last one has fossils in it. A Mach Bike and HM06 (Rock Smash) are required to get to the final floor of the tower.
You can only pick one fossil out of the two at the top of the tower. Once one of them has been taken, the other fossil will then disappear. It is possible to acquire the other fossil in the Fossil Maniac's extended tunnel later on in the game.