The subspace sinkhole was a gravitational anomaly in the Delta Quadrant that pulled passing ships into a pocket of space where time ran far faster, stranding Tuvok, Tom Paris and the Doctor in 2375.
The anomaly is the setting of the fifth-season episode Gravity. A shuttle carrying Tuvok, Paris and the Doctor's mobile emitter is drawn through the distortion and crashes on a desert planet inside, joining the wrecks of many earlier victims.
The castaways are helped by Noss, a resourceful woman who has survived there alone for many seasons and who falls in love with Tuvok. He, recalling the discipline he learned as a youth, will not return her feelings.
Because of the time differential, months pass for the survivors while Voyager searches for only a couple of days. An alien crew is about to seal the sinkhole permanently, and Janeway must beam her people out, together with Noss, before the pocket collapses.