Wherefore, because the soul is purified in this furnace like gold in a crucible…it is conscious of this complete undoing of itself in its very structure, together with the direst poverty, as if it were nearing its end, as may be seen by that which David says of himself in this respect, in these words: “Save me, O God, for the waters are up to my neck. I have sunk into the miry depths, where there is no footing. I have drifted into deep waters, where the flood engulfs me. I am weary from my crying; my throat is parched. My eyes fail looking for my God.” (Ps 69.1-3) Here God greatly humbles the soul in order that he may afterwards greatly exalt it. And if he ordained not that when these feelings arise within the soul they should speedily be stilled, the soul would die in a very short space. But there are only occasional periods when it is conscious of their greatest intensity…so keen that the soul seems to be seeing hell and perdition opened…and in truth [the soul] goes down alive into hell, being purged here on earth in the same manner as there, since this purgation is that which would have to be accomplished there.
(St. John of the Cross, The Dark Night of the Soul, Book 2, Ch. 6)