A subspace tunnel is a wormhole-like opening through space-time. Although both are topologically similar tubes of genus one and allow passage through distant parts of a universe or between different universes, the difference is where and how they exist. As opposed to the curvature associated with a wormhole, a subspace tunnel phenomenon exists solely in subspace and is powered by dark energy drawn from normal space-time. However, dark energy produces hyperphasic radiation within the tunnel, which can cause immediate disintegration of organic matter unless a properly harmonized multiphasic frequency is channeled into energy shields.
The Caeliar had found a way to create artificial subspace tunnels, and used them as part of their Great Work - the tunnels were used to remotely observe and potentially make contact with other species beyond their evolution. The tunnels were extremely stable, and remained after they had been created. The tunnels were opened by using a soliton wave, with the frequencies of each terminus being different. The USS Titan detected the tunnels during a scan of a remote area and stumbled upon the Caeliar planet New Erigol in 2381.
At some point prior to 2381, the Borg Collective located a subspace tunnel that connected a location close to their space with FGC-SR37-758, the supernova remnant at the former site of Erigol now deep within the Azure Nebula. They used this tunnel to launch an invasion of the Federation and surrounding galactic powers. (ST - Destiny novels: Gods of Night, Mere Mortals)