https://www.pcgamer.com/watch-fortnites-mysterious-purple-cube-melt/

The big news on the Fortnite front is, once again, the cube—the mysterious purple block that's been slowly working its way across the landscape. Today it finally took a much anticipated plunge into Loot Lake—and disappeared. 

Actually, what happened is that it seemed to melt, turning the waters of the lake a soothing pale purple before disappearing entirely. Then, a flash! And the lake sort of became solid—not exactly frozen, but clearly very bouncy. Strange symbols are visible just beneath the surface of the water, but what they mean is anybody's guess at this point.