The Cochrane inside an interfold layer
An interfold layer is a realm which exists in between space and subspace.
In 2373, the USS Voyager discovered an unstable interfold layer in the Delta Quadrant, while investigating a series of destructive astral eddies. It was discovered that the layer acted as a spawning ground for the eddies, which would erupt into normal space when they grew large enough. The layer was discovered when the crew sent a probe into one of the eddies, which collapsed back into the layer and took the probe with it. Although the probe could not be detected with sensors, it continued to transmit information on the layer to Voyager.
Later, while attempting to collect plasma particles left in the wake on one of the eddies, a shuttle piloted by Tom Paris was pulled into the interfold layer. Although there was a massive amount of plasma interference obstructing communications, Voyager was able to re-establish contact with Paris by adjusting the encoding filter and instructing Paris to calibrate to a theta-band frequency. Communications were further facilitated by widening the communications bandwidth. Paris was eventually able to return to normal space by piloting the shuttle into an eddy which was about to erupt, and riding it back. (VOY: "Real Life")