Aluminium (and pretty much all of its alloys) is incredibly reactive with air. This actually protects it because aluminium oxide forms on the surface in about a nothingth of a second which is then impervious to pretty much anything and protects the aluminium.
However, aluminium oxide dissolves
very
quickly in mercury (and just about nothing else). So, the mercury keeps stripping off the protective oxide, immediately after which the oxide re-forms from fresh aluminium - well you get the picture.
So, in a fairly short time, the mercury effectively eats its way through the aluminium. I've not tried it, but I'm told that with a spare bit of alloy sheet and a test tube of mercury a rather educational demo can be done.
G