Living creatures are amazing at building their homes from just about anything.
But sea-dwelling creatures are particular wizards: microscopic coccolithophores, coral-building algae, and giant snails engineer their own building material, like magic, by pulling two dissolved chemicals — calcium and carbonate — out of the water to form solid shells of, surprise, calcium carbonate.
The reason those shells don't dissolve back into calcium and carbonate as soon as they're built is that ocean water is already holding as much calcium and carbonate as it can, so the mineral forms much more easily than it dissolves.
At least, that's the way it works near the surface, where the shell-builders live.
But at greater depths, the water isn't quite as saturated with calcium and carbonate, and thus calcium carbonate is easier to dissolve.
So, unlike shallow coastal waters where shells of dead creatures build up on the seafloor, out in the deep ocean, there's a depth at which calcium carbonate starts to break apart and empty shells dissolve before reaching the bottom.
This "dissolving depth" depends on the concentration of calcium and carbonate already in seawater.
If the concentration is high, shells sink deeper before their calcium carbonate dissolves.
And if the concentration is low, the "dissolving depth" moves closer to the surface, meaning the deepest intact shells begin to dissolve.
But this is a feedback loop — shells that dissolve add more calcium carbonate to the water, making it harder for other shells to dissolve and lowering the "dissolving depth".
