Subspace compression anomaly

From Memory Alpha, the free Star Trek reference.

A subspace compression anomaly is a rare subspace astronomical phenomenon, that miniaturizes objects that enter its accretion disk. These anomalies emit gamma radiation.

In 2374, Starfleet ordered the USS Defiant to investigate one such anomaly, in the hopes of collecting data that could lead to the development of transwarp corridors. The runabout USS Rubicon was lowered into the anomaly with a tractor beam, where it shrank to a fraction of its normal size.

The mission was interrupted by a Jem'Hadar ambush. Contrary to predictions, the Rubicon did not return to normal size when it left the anomaly. After helping to defeat the Jem'Hadar in its miniaturized state, the runabout and its crew were restored to normal size by re-entering the anomaly along its previous course. (DS9: "One Little Ship")