The Lost River is a 31.1-mile-long (50.1 km) river in the Appalachian Mountains of Hardy County in West Virginia's Eastern Panhandle region. The Lost River is geologically the same river as the Cacapon River: it flows into an underground channel northeast of Baker along West Virginia Route 259 at "the Sinks" and reappears near Wardensville as the Cacapon. The source of the Lost River lies south of Mathias near the West Virginia/Virginia border. Along with the Cacapon and North rivers, the Lost River serves as one of the three main segments of the Cacapon River and its watershed.