[BlackMyron]

  I figured that they had a hand in it, but I'm curious how they managed it...

[Bradd]

There's been discussion of it in the forums here. If I recall correctly, they anchored magic to the world with giant pillars of orichalcum. Sort of a giant magical flywheel, which eventually came crashing to a halt.

[MJBurrage]

From what I've heard, the pillars didn't end up adding to the ambient magic, so much as they multiplied it.

That might sound better, but it meant that on the date when magic was due to go away, it still did, but with a crash rather than a whimper. (zero ambient magic multiplied is still zero magic)

I.E. without the pillars magic would have waned slowly, hitting zero on 12 August 3113 BC

With the pillars, the magic level stayed higher, but still hit zero on 12 August 3113 BC, taking Atlantis with it. (since the city, as built, could not stand without magic)

[Kot]

MJ, Atlantis/Thera needed magic to exist. So, it sank when the magic level finally declined and reached a low level which couldn't sustain it anymore. Definitely not 'level zero'. The end of the era marked by the sinking of Thera is a 'milestone' event, marking the end of high magic. Magic could be low then, just remember, there was a whole Fifth World during which it declined even further. So it couldn't be zero.
Plus, Atlantis/Thera was for sure a huge mana pocket.

[MJBurrage]

In my example I am calling the level of magic needed for simple spells, elf/dwarf births, and active dragons "zero"

This is the level that Earth went above on 24-Dec- 2011 AD, as it transitioned from the Fifth World to the Sixth World.  The same level that Earth went below on 12-Aug- 3113 BC.

As I understand it, the high power magics used by Atlantis should probably have stopped working well before the 3113 BC date, in the same way that high level magics should not have been possible in the Sixth World until decades after the 2011 date.  Things like the GGD were spikes above the ambient level, and Atlantis's pillars kept the local mana higher than normal as well.

However when the 3113 threshold date was reached, the pillars could no longer maintain the spells required by Atlantis.

[Kot]

You didn't specify this.

As for the pillars, it could be anything else - like sabotage, war, malfunction. Or it could be that they couldn't keep magic high enough to sustain orichalcum itself, and it was disrupted.