What is the difference "a" and "hasta"

Question

I was reading through some funny pictures and one decía "Metí una galleta en un vaso de leche. Se puso demasiado blanda y se hundio hasta el fondo." Why can't you say "Se hundio al fondo" ?

Answer 1

It can be said like that, it'd be correct.

The difference is just that "Hasta el fondo" is more descriptive than "Al fondo", plus it's more common to say "Hasta el fondo".