An Underspace corridor.
Underspace corridors are the name for a group of subspace corridors, an apparently naturally occurring in some regions of the Delta Quadrant, and possibly beyond. They appeared to be analogous to a naturally occurring version of the Borg transwarp network.
In the 15th century, the underspace corridors allowed a species known as the Vaadwaur access to remote regions of the Delta Quadrant, as far from their homeworld as Talax. The Vaadwaur used these corridors for trading and mercantile activities, as well as colonization, including imperialistic activities. After the Vaadwaur were defeated, another species, the Turei, claimed the corridors as theirs. The Vaadwaur extensively mapped these corridors, and claimed that the network extended much farther than the Turei or anyone else know of.
In 2376, the Federation starship Voyager passed near one of corridors, and interactions between the corridor and their warp drive accidentally pulled them in. (VOY: "Dragon's Teeth")