The Wellspring is the origin of Magic in the whole universe as well as the foundation of Fillory. It's a mystical energy stream with boundless magical properties. If drunk, this source gives great power to magicians, but at the expense of all others who find themselves with fewer source to draw on for their spells.
The source is sealed by a circular golden door with alchemical and astrological symbols inlaid in concentric circles. To open this seal, it's necessary depositing a magic marble in the middle of the concentric circles. However, the magic marble is scorching at the touch of inexperienced magicians, and only Gods or magicians "touched by Gods" can handle it, such as Alice Quinn.
Martin Chatwin, eager to learn major spells in the shortest possible time to escape the abuse of Christopher Plover, cast a spell to find the Wellspring of magic and began to drink its water. This made him an almost all-powerful magician, but also caused the rapid deterioration of the balance, causing the premature depletion of magic which is starting to disappear from the world.