Some times it will be there, some times it won't. Just keep checking it at different times until it shows up.
Once you get in it, there are some of those mach bike puzzles (which I detest).
At the top are two fossils, of which you pick one and the other one you can get after you beat the elite 4.
After you spoke with the fossil maniac
mirage tower only appears in pokemon emerald.
The first fossil appears at the desert the second appears in mirage tower.
Mirage Tower is in the desert around the enterance near Mauville. It only appears every once in awhile. I'm not sure how often though.
From Prince Kayne:on certain days/times a tower appears the Mirage Tower near the southern entrance of the desert.
In Pokemon Emerald, The Mirage Tower appears randomly. All you have to do to make it appear is keep going back-and-forth from Fiery Path to the Desert and it should appear. Just make sure you have the Mach Bike, You'll need it inside the tower to get past the fragile sand floors. And at the end of the Mirage Tower, you'll get a nice suprise! :)
Armaldo and Cradilly.
In the desert and in mirage tower.
they are at the top of mirage tower the tower in the desert north of mauville city
go into the desert,and find the mirage tower
mirage tower or desert underpass
In the Mirage Tower nefore you get the Fossil
In mirage tower ( at desert near route 111)