USS Voyager approaching an astral eddy.
An astral eddy is a type of dangerous spatial disruption that forms at the confluence of space and subspace. It manifests as a whirlpool of plasma with a temperature gradient of nine million kelvins, and gives off massive discharges of plasmatic energy. There is a calm "eye" at the center, like that of a hurricane, where matter is exchanged between space and subspace.
The USS Voyager encountered this phenomenon in 2373, when one destroyed a Vostigye science station. Subsequently, a type 9 shuttlecraft piloted by Tom Paris was drawn into an eddy, emerging in the interfold layer where the eddies form. Paris managed to return to normal space by riding the leading edge of a newly forming eddy, surviving long enough to be beamed to Voyager. (VOY: "Real Life")