"Deep calls to deep"

At our work prayer group this week we were looking at Pslam 42 (As the deer…), and we all found ourselves tripping up over verse 7:

Deep calls to deep in the roar of your waterfalls;

all your waves and breakers have swept over me

On one level I love this verse, as it’s so evocative – Matt Redman certainly thought so too, but when you actually stop and take stock, it’s not at all clear what it actually means. Perhaps we’re used to depth being a positive thing; “how deep is your love?” or “she’s such a shallow person” – yet there’s also being “out of your depth”…

The context suggests the pslamist is somewhere between desperation and despair at this point, and one commentary I read said that this verse talks about the depths of despair, and misery upon misery being piled upon him.