(covers information from several alternate timelines)
A quantum fissure is a spacetime anomaly which acts as a fixed point across the spacetime continuum, described by Lieutenant Commander Data as a "keyhole" which intersects many quantum realities. In a sealed state an anomaly of this type remains undetected and presents no threat to the continuum.
A fissure was encountered by Lieutenant Worf while returning to the USS Enterprise-D from a bat'leth competition on Forcas III. When Worf's shuttlecraft Curie came into contact with the fissure, its warp engines caused a small break in the barriers between quantum realities. Worf was thrown into a state of quantum flux, shifting him into other realities. The quantum flux in Worf's cellular RNA was intensified by the subspace field pulse of Commander La Forge's VISOR, pushing Worf into a new reality every time he was in close proximity to La Forge. The shift caused him a momentary feeling of dizziness, similar to the side effects of a concussion.
Although the fissure could not be seen, Data was able to enhance it with a warp field to make it visible. Using a subspace differential pulse, Data was able to scan over ten million different quantum states within the fissure, but was unable to locate the one which shared Worf's quantum signature. Later, due to an energy surge in the subspace pulse, the fissure destabilized and the barriers between quantum realities began to break down, causing approximately 285,000 quantum incursions by the Enterprise-D to appear from other quantum realities. Eventually the quantum barriers of all the realities intersected by the anomaly would have collapsed and filled the sector where the anomaly was located.
By identifying the Enterprise which shared Worf's quantum signature, they were able to appropriate the same shuttle which Worf had used to traverse the fissure in the first place. By piloting the shuttle into the fissure and initiating a broad-spectrum warp field, Worf was able to seal the fissure before it completely broke down. This also had the effect of transporting Worf and the shuttlecraft back in time to a point before Worf encountered the fissure. Due to the uncertainty principle, Worf could have emerged back into his reality several days before or after the event. (TNG: "Parallels")