When king Arthur threw the sword into the lake, he did so so that nobody else could wield it, because the dragon who forged it told him that he could be the only person ever to use the sword. Now, the sword disappeared in the lake, as the Lady of the Lake took it to keep it safe. To me, all of these elements seem sort of supernatural, so I'd say that is the element that the disappearance of the Excalibur into the lake represents.
