The Children of Atom believe that this spring was blessed by Atom. They send those who wish to join their ranks to drink from the spring, knowing full well that the spring is poisonous. They believe that those who survive are capable of serving Atom. Those that return are welcomed into the cult, however, some suffer brain damage as a result.
It is a highly irradiated spring where the Sole Survivor gets sent during Visions in the Fog, to drink out of it to receive "Atom's Gift."
Visions in the Fog
Drinking from the spring before obtaining the quest Visions in the Fog will add radiation damage, but the Sole Survivor will not see the Mother of the Fog until they begin the quest.
↑The Sole Survivor: "That spring was poison. Were you trying to kill me?" Brian Richter: "Those waters are blessed with Atom's holy Glow.......which you survived. Was that the extent of your experience?" (Richter's dialogue)
↑Theil: "Glory to Atom, brother/sister.You must be the one Richter sent out to the spring. Don't appear to have turned into a slobbering goon, so I'm guessing you had a favorable experience. What visions did holy Atom bless you with?" The Sole Survivor: "Slobbering goon? Do people have bad reactions to the spring?" Theil: "Few make it through the ritual intact. Sort of the point. A vision from Atom is a great honor. Not something to be granted to the unworthy.So tell me, what did you see?" The Sole Survivor: "So the spring is supposed to hurt people?" Theil: "The spring shows who is capable of serving Atom and who is not. Clearly Atom saw your strength and blessed you with a vision of..." The Sole Survivor: "A woman. She guided me to a small shrine. The Grand Zealot said she was called "the Mother of the Fog."" Theil: "Heh. You. Saw the Mother of the... how did...I guess it's not our place to question the ways of Atom. If there is anything I or the other Zealots can do to serve, all you need to do is ask." (Theil's dialogue)