The Well is a location featured in RAGE. The entrance is located in the "Wellmasters" in Wellspring. It is what the citizens of Wellspring use to purify their water supply. At one point in time it was a sewer of some sort, but has since been cleared and used as a purification facility.
The protagonist is forced to travel there during the quest "Hijacked Well" when the Ghost Clan takes it over. Carlson, after triggering the alarm signifying there is a problem, will explain that the Ghosts have planted some sort of toxin into the water supply and that it could cripple the whole town which he cannot allow to happen.
The player can only enter the Well during "Hijacked Well".