https://www.gamespot.com/articles/the-fortnite-cube-sank-into-loot-lake-turning-it-p/1100-6461891/

With Season 6 on the horizon, the saga of the mysterious cube in Fortnite: Battle Royale has seemingly reached its final conclusion (for now). The object first appeared earlier this season, and it's now made its way to Loot Lake. Once there, it either sank or melt into the water, altering the makeup of the entire area.