Subspace fracture
A subspace fracture is a phenomenon which can be created as a byproduct of a polaric ion explosion, and which can allow for travel through time. Kathryn Janeway described them as "fractures in time".

In 2371, the  discovered a planet in the Delta Quadrant whose inhabitants had all died in a polaric ion explosion. The shock wave from the explosion, unlike most shock waves which dissipate into the future, dissipated into the past, leading to a chain reaction that shattered subspace. This created subspace fractures at the moment of the explosion that then floated across the planet's surface like icebergs. The time of the explosion represented the widest point of the fractures, but they closed as time passed. Investigating, Captain Janeway and Lieutenant Paris became caught in one of these fractures, and were transported back in time to the day the explosion occurred. In an attempt to rescue their crewmates, the Voyager crew constructed a device that could successfully widen a subspace fracture for Janeway and Paris to return through. 

As the subspace damage dissipated the subspace fractures closed, making a rescue attempt more difficult. Returning to the planet, they activated the device and opened a fracture at the location of the explosion, which Janeway observed in the past. Noticing that, as the fracture grew larger, it was in danger of intersecting with a conduit wall in a polaric ion power plant, Janeway realized that it was the Voyager crew's own rescue attempt which caused the explosion in the first place. Using her hand phaser, Janeway managed to seal the fracture, thereby preventing the explosion and changing history so that Janeway and Paris were never transported back in time to bring about the explosion in the first place. ()

See also
Space-time fracture

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fracture
Time travel