The Well

The Well is a quantastital multi-phasic spatial anomaly with pan-dimensional and temporal rifting properties located in the same sector as Starbase 6. In the early days of space travel through that sector, The Well was responsible for the disappearance of at least a dozen ships. But by the 23rd Century, it was completely charted and sitting near heavily-traveled routes deep inside Federation space. The Well is actually a junction between universes and has end points in many different ones in many different time periods.

Dr. John Smith took the SS Clydesdale and its crew through The Well to return to his own universe and gave them coordinates to return safely home. ("Back to the Well" [TOS])