This is a simple page about the Southern Island, because I have been recieving many emails about it. Southern Island is very small, and because of that it does not show up on your map. To get there, you need an Eon Ticket. Ways you can get the Eon Ticket:
At special events, such as at the Pokemon Center in NYC
Using the E-Reader card which came with issue #173 of Nintendo Power
Or by Swapping Data with someone that already has it.
Please note that unless you do something like this (or use a cheating device, which I do not advice), the Eon Ticket cannot be obtained through normal in-game methods.
About the Eon Ticket
Once you get the Eon Ticket, talk to your father in Petalburg gym. Also note that you must have beaten him to do this. He will tell you a letter arrived for you and that you must go to Lilycove. Once there, you show it to the lady at the port and she will bring out the Captain. He tells you that you don't know what exactly you'll find on an unexplored island such as that.
Southern Island
Once you get there, there is not much space to look around. You will enter a forest and see some water scattered around with something that looks like an egg. Once you talk to the egg-shaped thing, the Latios or Latias which is not in your game, will fight you at level 50. However, be aware that you can only fight Latios/Latias once. In addition to fighting the one which is not in your game, it will have a special item attached to it if you catch it. It will have a Soul Dew, which increase Latios, or Latias's Special Attack and Special Defense by 1.5x if it is attached to it in battle!
Now that I wrote a page about this, hopefully this will lessen some of the emails in my inbox. ^_^;