A vast network of subspace corridors in the Delta Quadrant, controlled by the Turei, that allowed travel across hundreds of light years in minutes.
Underspace is the name the Turei give to a network of corridors running through subspace, first encountered in Dragon's Teeth. Voyager is caught in one and travels more than two hundred light years in a few minutes, but the Turei, who guard the corridors closely, demand that all record of them be wiped from Voyager's computers.
Centuries earlier the corridors had been discovered by the Vaadwaur, who used them to invade other worlds until their rivals drove them into hiding. Voyager's interest in the corridors leads Seven of Nine to revive Gedrin, setting off a renewed conflict between the Vaadwaur and the Turei.