i've been over it before, and i'll go over it again: the sky.
actually, ABOVE the sky.
when god creates the earth in genesis 1, first he creates heaven. a firmament which separates the waters below from the waters above, dividing the great deep in which everything exists. the waters below collect to form the seas.
when god floods the earth, he is literally and figuratively and sybolically un-making creation. so god punctures this firmament opening "the windows of heaven." and out pours water. and he opens the fountains of the deep (the water below, possibly).
you'll hear a lot of christians reply to this and debate it. but it's what the bible says, and is consistent with the worldview of the time of every single culture in the area. it is consistent with the thought that water was the element from which all was made, also common to every culture in the area (as well as many others).
אָרַח