Graviton ellipse
From Memory Alpha, the free Star Trek reference.
Graviton Ellipse (designated as Spatial anomaly 521 by the Borg) was a stable pocket of normal space moved by a surrounding elliptical gravimetric distortion perimeter measuring millions of terajoules of subspace energy.
Graviton ellipses traveled through subspace. Emerging into normal space and extra-dimensional realms only when they were in proximity of objects that emited electromagnetic energy, such as spacecraft and dark matter asteroids. The ellipses themselves generated an electromagnetic radiation field, of dangerous levels to humanoids, that made the anomaly react like a magnet drawn to another magnet, relative to it's target. The Borg had discovered that cutting down power and simultaneously reversing the polarity deflector shields will result in the anomaly no longer being attracted to the ship. The anomalies remain outside of subspace for some hours only.
In addition to the electromagnetic field these anomalies emited level 9 gravimetric distortions, through subspace to the space around them, creating severe turbulence. As an ellipse emerges from subspace, the initial target it was drawn to is already in range of the distortions and will no longer be capable of forming warp fields to escape. In addition, the target ship will experience a ship-wide power drain. An ellipse could overtake the ship even if it was traveling at maximum impulse.
The stress caused by the gravimetric interference was higher closer to the perimeter and will tear some starships apart. Some ships could however survive the ordeal intact. The actual elliptical perimeter of the anomaly was composed of gravimetric currents of increasing gravimetric shear intensity closer to the interior of the phenomenon. As the anomaly changed course or devours objects it discharged gravimetric surges in the perimiter that travel outward and even affected objects thousands of kilometers away.
The core was totally free of gravimetric distortions. The electromagnetic activity of the anomaly created a natural luminescence and the ambient temperature of the interior was -260 degrees. The pocket of normal space remained completely stable and calm regardless of what the speed, location or movements of the anomaly were. The Borg have theorized it to be possible to dissipate the anomaly from within its interior. To that end, they had developed shield enhancements to allow ships passage through the gravimetric distortions. These enhancements could also be implemented to shields used by Starfleet craft such as the Delta Flyer or a class 2 shuttle. Modifications took several hours and required an operator familiar with Borg technology to succesfully maintain the protection.
One such graviton ellipse, with a perimeter over a kilometer in diameter, had been in existence from the time of the formation of the Milky Way Galaxy. It was found to have devoured objects from all of the quadrants of the galaxy and from extra-dimensional realms. Even an atmosphere had formed from the interactions of chemicals it had accumulated. In 2032 this anomaly emerged in the Sol System and devoured the Ares IV Mars orbiter. This was mankinds first encounter with a spatial anomaly. The same ellipse attempted to devour USS Voyager, in 2376, in the Delta Quadrant. Commander Chakotay suggested the anomaly to be renamed as the "kitchen sink anomaly" due to the debris field contained inside.
Graviton ellipses have been observed only a handful of times, and no known ship has ever survived the encounter or escaped being devoured, before the Voyager. (VOY: "One Small Step")
