Southern island

It does exist. You need to get the Eon Ticket from a Nintendo event (Action Replay will not work). It is where you catch Latios (or in Ruby, Latias).

I am going to have to disappoint you... Yes, southern island exists... But you can only get there by attending a long since over nintendo special event (usually in japan not long after a Pokemon game has been released)

You can't get there anymore without cheating using an action replay...but there is one other way... Without cheating... I will mention that later.

There is a special event pokemon... It is the sibling of latias/latios (it depends on whichever one you chose to find roaming around hoenn after defeating the E4 or if it's ruby or sappphire the other one you didn't get)

Really, since it is actually much easier to get two game carts, one of ruby one of sapphire (or add one of emerald in there, in emerald, you get to choose which lati you can catch) and catch one of each in those games, then just trade the Pokemon over to your 'home' game... The game cart you always play (either own two gameboy advances for this, or borrow a friend's for a few minutes...)

The other way you can get to southern island, is by mixing records with a (ruby or sapphire) person who has a LEGITIMATE eon ticket... (it can't be cheated) and then you will notice after having done so, that the eon ticket has transferred to you.

Then go to the lilycove boat house, and talk o the girl there, and she will get the captain who will take you to southern island.

There you battle the lati!

Southern Island does exist. I forgot it's exact location on the map. You can only get there by cheating. There, you can catch the other lati. On Sapphire, you can go there to catch Latios. On Ruby, you can go there to catch Latias.

Southern island does exist but only with an Action Reply or a GameShark.
Other than that it doesn't.

To get to southern island you need to have the Eon Ticket which can be Obtained
With a Cheating Device or through a Event.
When you have the Ticket you go to LilyCove to the Harbor and talk to the Lady.
If she say's the same thing as all the time then the ticket didn't work and thuse you need to activate it with a cheating device.

If it works she will say "Oh...that ticket" and she will walk into the Ship and out will come the Sailor and he will talk to you about an island know as Southern island and that he can take you there so you will go there.
.:SAVE BEFORE YOU GO THERE:.

When you get there just walk up to the entrance to the forest whatever it is.
In there you will see a Small Rock sorta egg shap and go to it and press "A".
Also Save it before you do.
Then you will hear a noise and down will come Latios if you have Sapphire and if you have Ruby down will come Latias and then you batttle them I think there Level 50 when you fight them there.
So yeah thats pretty much that.

You have to downlond a specel ticket to get there but I don't now how to do that sorry