How did magic return to the world?

The following information was discovered by historian Mortimer Price and revealed to the world by the legendary musician Hunter Chordato:

In days of old (long before the Common Era), there was ambient mana in the Earth’s atmosphere, which explains ancient legends of magic, sorcerers, witches, magical beings, etc.  At one point, however, a group of sorcerers discovered a massive cavern filled with gigantic quartz crystals in the region that would later be known as France.  Hoping to enhance their own power, the sorcerers enchanted all the cavern’s crystals to absorb and store ambient mana, with the goal of transforming the cavern into a giant mana battery that the sorcerers could tap at will.  Over the years, as the crystals slowly drained mana from the atmosphere, it became harder and harder for casters outside the cavern to perform spells (i.e., the spirit mana cost for spells kept getting higher and higher–but no one knew why).  Progressively, the more powerful spells became impossible to cast and, by Roman times, the ambient mana in the atmosphere had become so minimal that it was virtually impossible to perform anything other than minor magic tricks.

In approximately 40 BCE, a group of Roman soldiers discovered the cavern and were set upon by the cavern’s inhabitants, who were still able to use magic at full power.  The battle was brutal, fiery, and bloody.  Eventually, in desperation, the Romans smashed one of the crystals, causing an explosion of deadly energy that wiped out all of the Romans and all of the sorcerers and sealed the cavern (without actually damaging the cavern’s crystals).  Because the crystals were still enchanted, they continued to drain ambient mana until there was almost no mana left in the Earth’s atmosphere.  This is the world that everyone in the pre-Great-Destruction Common Era came to know and the tales of ancient magic were simply dismissed as myth.  When the Great Destruction happened, however, a very powerful nuclear weapon detonated in close proximity to the cavern, shattering the crystals and releasing all of the captured mana back into the atmosphere.  Thus, ambient mana went from 0% to 100% in the blink of an eye.  And, since the enchanted crystals were destroyed, the ambient mana was no longer being drained away.