Underspace

The Underspace was a network of subspace corridors that extended through a large area of the Delta Quadrant, allowing starships to traverse vast distances in a short time.

Prior to the 15th century, the Vaadwaur discovered and charted the Underspace, using them to travel throughout the Delta Quadrant, as far as Talax and beyond. Though they engaged in trade and exploration, they also used the corridors to mount lightning raids on other species and to expand their territory. The Vaadwaur kept no written records of the Underspace, rather keeping the information committed to memory.

Eventually, the Turei and a dozen other species formed a coalition and eliminated the Vaadwaur threat, rendering the Vaadwaur homeworld uninhabitable. They then claimed the Underspace for themselves, though they lacked the Vaadwaur's knowledge as to how to maintain it.

In 2376, the USS Voyager's warp field interacted with one of the corridors, and it was pulled through a breach in the radial wall. The Turei expelled them from the corridor with a resonance pulse, but then demanded to board Voyager to eliminate all evidence of the Underspace. Voyager took shelter on the radioactive Vaadwaur homeworld, where they discovered a group of Vaadwaur who survived the war in stasis pods.

The Vaadwaur offered to cooperate with the Voyager crew and show them corridors of which the Turei have never been aware. However, their desire to regain their former power ultimately led them to attempt to capture Voyager and its advanced technology. Voyager defeated them with help from the Turei, and the remaining Vaadwaur ships fled into the Underspace. 

アンダー・スペース