A micro-wormhole.
Interior of micro-wormhole.
A micro-wormhole was a wormhole of much smaller size than their counterparts. A micro-wormhole could have originally been of a much greater size, but was in an advanced state of collapse, and over time had degraded and shrunk to a smaller size.
In 2371, the USS Voyager discovered a naturally-occurring micro-wormhole thirty centimeters in diameter. The wormhole led from the Delta Quadrant to Romulan space (in the past). The crew gave the wormhole the name Harry Kim wormhole. (VOY: "Eye of the Needle")
In 2376, the Pathfinder Project at Starfleet Communications was able to create a temporary, artificial micro-wormhole using the MIDAS array, to briefly communicate with Voyager over 30,000 light years away in the Delta Quadrant. (VOY: "Pathfinder")