Seems like a good a place as any to just drop a quote:

Quote Originally Posted by Encyclopaedia Eorzea
Far to the south of the Three Great Continents, beyond the equatorial line, lies the second of Hydaelyn's three landmasses - Meracydia. Five millennia ago at the close of the Third Astral Era, the Allagan Empire, unsatisfied with controlling merely Aldenard, Ilsabard, and Othard, set their sights to the south and began an invasion that would end with nearly the entire continent being rendered a wasteland. While some areas remain uninhabitable even today, those which have been repopulated remain recluse and very rarely welcome contact with the civilizations of the northern hemisphere, ofttimes attacking any who approach, whether it be by sea or by air. Because of this, very little of the region is known, including even the most basic of geographical information.
That said, if I remember correctly, there are mentions of trade expeditions to Meracydia, so they can't attack any and all who approach, apparently.