In an alternate timeline, a rupture was formed as the result of the convergence of three identical inverse tachyon pulses from three time periods - 2364, 2370 and the early 25th century - at a single point in space. The rupture, located in the Devron system inside the Romulan Neutral Zone, expanded as it traveled back through time due to the unique nature of anti-time. Eventually, the anomaly would have expanded to cover the entire Beta Quadrant in the distant past.
A highly-focused energy source, the anomaly's energy output was found to be equivalent to ten G-type stars. The energy emitted was in the form of temporal energy, which caused Geordi La Forge's DNA to regenerate, giving him normal sight. Several other crew members reported old scars healing, and nurse Alyssa Ogawa's fetus was killed by the reversion. The anomaly's presence at Earth in the distant past would have prevented the formation of the first proteins, and ultimately, all life on the planet.
Eventually, the cause of the rupture was uncovered, and a concerted effort from the Enterprises of each time frame was successful in reforming the subspace barrier via a static warp shell, erasing the anomaly and the timelines it created. (TNG: "All Good Things...")