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. Those classified as Master Magicians may also be able to interact with the lock and key of the wellspring. The Wellspring appears to be self-replenishing according to statements, implying Magic is endless as long as the Wellspring is functional.
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.
The power of the well is also sought out by the neighboring kingdom of Loria, who wants access to it.
Ember later defacated into the spring, causing massive damage as magic became unreliable and Fillory suffered various "brown outs", in which spells and magic failed. In desperation, Margo made a deal with fairies to clean and restore the spring completely. Despite being the original source of Magic, it appears certain beings such as Gods still can use Magic even if the wellspring dies. Example: Reynard was able to cast Martin's paralysis spell during a wellspring black out. Also with extreme work and years batteries can be made to contain magical current. Certain Magical Creatures retain their Magic even if the Wellspring doesn't give Magic. However Niffins—as they draw power from it like Magicians—slowly die as a result of Magic being gone.
- The Wellspring apparently has a filtration system according to Ember.
- According to Christopher Plover, The Wellspring's Magic is so strong it destroyed everything human in Martin. However Martin alluded to removing his Shade himself. It's unknown if Plover was correct or that Martin removing his Shade and drinking from the spring ;were two separating insidious acts or the same thing.