Once a day, you will find a random island on the outskirts of the Hoenn Region. These islands are relatively small but can be seen due to a distinctive red shine above them. These are the Mirage Islands. There are five different varieties: Mirage Island, Mirage Forest, Mirage Cave, Mirage Mountain & Crescent Isle, each with different attributes, and each island in each variety is different. As each day passes, one island disappears and the next island re-appears
When you get to the island, you will find one or two items, with some rare items such as the Deep Sea Tooth being hidden with the Dowsing Machine. Some islands also house TMs for you to find. In addition to that, many of them hold a variety of wild Pokémon, most of which are not readily available in the Hoenn region making them the only place you can get them.
Another aspect of this that these islands use the StreetPass feature. If you have seen the island for the day and you then StreetPass another player, that player will then be able to see and land on that particular island and you will be able to see and land on the one they have, in addition to the one you have as standard. This also works through going online. Any players you Pass By will pass their Mirage Islands to you.
With the items, the hidden items only appear one a day and at random between a selection of three. These also will only appear in the area that's indigenous to your game for the day, and not through any found via StreetPass or PassingBy. The Dowsing Machine items available are based on the island type and are as follows