From The Noonday Demon by Andrew Solomon

Depressives use the phrase “over the edge” all the time to delineate the passage from pain to madness. This very physical description frequently entails falling “into the abyss.” It’s odd that so many people have such a consistent vocabulary, because the edge is really quite an abstracted metaphor. Few of us have ever fallen off the edge of anything, and certainly not into an abyss. The Grand Canyon? A Norwegian fjord? A South African diamond mine? It’s difficult to even _find_ an abyss to fall into. When asked, people describe the abyss pretty consistently. In the first place, it’s dark. You are falling away from the sunlight toward a place where the shadows are black. Inside it, you cannot see, and the dangers are everywhere (it’s neither soft-bottomed nor soft-sided, the abyss). While you are falling, you don’t know how deep you can go, or whether you can in any way stop yourself. You hit invisible things over and over again until you are shredded, and yet your environment is too unstable for you to catch onto anything.