I have known this feeling for years. Sometimes it sneaks up, the water cooling and deepening gradually, walls coming up ever so slowly around. Sometimes it strikes full-strength without warning. I was too surprised to be terrified by it the first time. I was devastated the second time, because I knew it wasn't going to go away. I lived in dread, after that, waiting, holding my breath, so I wouldn't be caught off-guard again. It's been more than ten years now, since the first time, and I've learned to cope, learned who to turn to when the world has ended and I'm trapped alive under the wreckage. Most days are normal, even happy.
It's perhaps worse now that I have healed my mind enough so that these attacks are few and far between. When I was living every day under the battleship-grey banner of long-term depression, the contrast between the two states wasn't so vivid. The change between adrift in a stormy sea with no land in sight and nothing to cling to and the poorly-lit well is a small one, and sometimes abject misery can be relieved, somewhat, by a sudden struggle between life and death with one's mind as the battleground, oneself the sole chosen champion. But when life is decent, and the moments that aren't an extreme of any emotion are a vague contentment and sense of well-being, it's a rude shock to suddenly find oneself in the midst of a mental battle that could well prove fatal.