"Have respect unto the covenant: for the dark places of the earth are full of the habitations of cruelty." (Psalm 74:20)
The hand in front of your face
The ocean is layered. Five levels of depth have been taxonomized by scientists, the bottom of which, is pitch black (So's the one above it.). That one's called the hadalpelagic layer (The fourth being the abyssopelagic.). In the broadest sense, I'm reminded of both the earth, with its constituent crustal layers, and also our lives. It's the latter with which I would like to view the whole "rising and falling" motion as it applies to our days and yes, our lives. Now, in order to let the light in, God has to withdraw the waters. He'll walk you through but you have to hold on to His hand. The normal directional skills don't apply down there. They break down and don't work. This is true.
"Thrice was I beaten with rods, once was I stoned, thrice I suffered shipwreck, a night and a day I have been in the deep." (2 Corinthians 11:25, emphasis mine)
So, the top three layers of ocean are as follows: The sunlit, twilight, and dark zones respectively. And it's one thing to splash around in the tide pools on the shore and only watch the breakers roll in and recede. God would have you go out as far as you please. But He also wants to introduce you to depths of which you will never see on your own. There's this seeming dichotomy to life. This is why we "take up [our] cross daily, and follow [Jesus]." (Luke 9:23b) If Jesus invites you to join Him at the bottom of the ocean--after He's split the sea, of course--for a picnic or some such, it's the chance of a lifetime and best to follow Him out there.
The hand holding yours
"A friend loveth at all times, and a brother is born for adversity." (Proverbs 17:17)
And this is where the first part of the first verse comes into play. Asaph says "Have respect unto the covenant". God will not forget His Word to you. If there are specific things He's spoken and intimated to your heart for the season and/or layer in which you find yourself, feed off them. But know that everything spoken by Him in the Bible is for you, at all times. And it's those times at crushing depths that you'll feel everything you knew about God accosted. Simple problems of doubt and belief and love and patience will be the battlegrounds. It shows how important they are if only for the deep crisis you feel when once your basic belief in the God you know and love (and who loves you) looks like supreme and illogical folly.
For me, there's a lot of life that comes back around. I will go through a stage or a certain-feeling season and bid it goodbye (sometimes consciously, sometimes unconsciously) and forget about it. And then it'll crop up again. I'll begin feeling the same struggles and intuit the same answers to the problems that (now) recur. It would seem there are pairs to many of the things God wants to show me. Does this mean that I somehow failed the test the first time? No. I don't really know what it means. But I do find myself going down to the bottom and upon my resurface to the layer of "daylight", feel stronger and more assured. As if I brought the strength needed to survive in the hadal abyss, into the mundane. That's priceless.