Subspace compression anomaly

The subspace compression anomaly was a phenomenon that was discovered in Federation space in the 2370s. It is believed that they are caused by either the fusion of black holes and antimatter black holes or the intersection of subspace fissures and temporal distortions. 

In 2374, Starfleet assigned the USS Defiant (NX-74205) (I) and the USS Rubicon (NCC-72936) to investigate the anomaly in the hope it could help in the development of transwarp corridors. The Rubicon entered the anomaly, tethered to the Defiant by a tractor beam and began to shrink. The mission was halted when the Defiant was attacked by the Jem'Hadar and the tractor beam holding the Rubicon was lost. After helping the crew to regain control, the Rubicon and its crew returned to the anomaly and were restored to their normal sizes. 

Starfleet later deployed warning buoy Delta 917-Kilo to warn travellers of the subspace compression anomaly.