The Mirage Tower is one of the new Areas discovered in Pokémon Emerald situated in the Desert

It only appears after talking to the Ruin Maniac in Fallarbor Town. Head back to the desert and it'll be here. Be Warned though, you need the Mach Bike here as, like the Sky Pillar, it has Cracking Floor Puzzles
You should also bring Rock Smash and some Water Pokémon to fend off the Wild Pokémon
Once you get to the top of Mirage Tower, you will see the two fossils. Pick One and the Pillar will start shaking
Mirage Tower will disappear forever, however the other fossil falls out and sinks into the Sand. However it is not lost

Wild Pokémon in Mirage Tower

Here are the Pokémon you will find in the wild in Mirage Tower:

In The Pillar
	Trapinch	Common
	Sandshrew	Uncommon