A warp bubble, also known as a static warp bubble or static warp field, is a type of subspace deformation. Some individuals, such as those native to the planet Tau Alpha C, are capable of manipulating warp bubbles with their minds.
In 2367, Ensign Wesley Crusher performed an experiment aboard the USS Enterprise-D, based on Kosinski's warp equations, wherein he created a warp bubble within the warp drive of the Enterprise. The bubble remained roughly stable at a field distortion level of 1,283 x 3,695 millicochranes. During the experiment, Wesley's mother, Doctor Beverly Crusher, became trapped inside the warp bubble. She was esentially caught in her own parallel universe, where her thoughts at the very moment she was trapped shaped her reality. As Dr. Crusher's thoughts were of losing old friends, her reality manifested itself as one where the crew of the Enterprise disappeared one by one, eventually leaving her alone.
As Wesley and the Enterprise crew attempted to retrieve Dr. Crusher before the bubble collapsed, they were visited by the Traveler, a Tau Alpha C inhabitant, who assisted Wesley in keeping the bubble stable long enough to create a threshold between both realities through which Dr. Crusher could escape. In the bubble, as Dr. Crusher began to realize what was happening, she encountered the threshold Wesley was creating, which manifested itself as a violent atmospheric vortex. As the bubble was on the verge of complete collapse, Dr. Crusher jumped through the vortex, returning to her own reality. (TNG: "Remember Me")
Trionic initiators in warp coils have been known to unintentionally produce warp bubbles and other subspace deformations when they are adjusted. Such initiators were used on the USS Hera. (TNG: "Interface")