The Southern Cross is a discovery related to the purple continent area.
A mysterious, cross-shaped island. The building that sits atop it is said to be a remnant of the Purple Civilization. Legends say that the long arm of the cross points toward the location of that civilization, but no one has ever confirmed it.
It is located in Deep Sky, off of the northern border of the Lands of Ice, east of the field of icebergs. Because of this, one must have the Purple Moon Crystal to actually discover this discovery. It is visible before it is discovered, so players simply need to search the area until they find the island.
The cross's long arm points directly to the lower entrance of Glacia, which can actually be entered from the lower area. This can be used in reverse to find it by starting from Glacia and heading directly north until the island is encountered. Note that if you cross the south ocean sky rift or enter the south ocean area, you have gone too far north.