Mana -- the raw stuff of magic, the source of a unicorn's spellcasting powers, and a force that empowered earth ponies and pegasi in their own ways as well -- was generally thought to be spread just about everywhere, in fairly even quantities. It seemed to just be a natural part of the universe, an ethereal field that pervaded reality. There were some places where it was maybe a little stronger, and some places where it was a little weaker -- the latter, often the result of magic spells gone wrong. 