A large and spectacular cave in Germany Valley, Pendleton County, Hellhole occurs in middle Ordovician limestones along the axis of the Wills Mountain anticline. The entrance, consisting of three openings in a large sink, is a sheer drop of 160 feet into an enormous chamber with a floor expanse exceeding one acre. In November 1929, a team led by Arthur E. Krause, a Lutheran pastor and president of St. John's Academy in Petersburg, made the first recorded descent, which was accomplished by means of a windlass and steel cable. They explored several thousand feet of spacious passages, all that would be known for the next 40 years. By the 1940s, Hellhole and nearby Schoolhouse Cave were probably the most famous wild caves in the United States.

Since the early 1970s, National Speleological Society explorers have expanded the mapped passage length of Hellhole to 22.1 miles, with a maximum depth of 519 feet. Many of the cave's rooms and passages are quite large and often profusely decorated with stalactites, stalagmites, and other features. Presently the longest cave in the state north of the Pocahontas-Greenbrier line, Hellhole is still incompletely explored. Many cavers believe that its aggregate length may exceed 50 miles.

Hellhole is one of the most important bat caves in the East, containing a hibernating colony of more than 100,000 little brown bats as well as large numbers of endangered (and federally protected) Indiana and Virginia big-eared bats. In 1981, the U.S. Fish and Wildlife Service, with landowner cooperation, fenced the entrance sink to prohibit unauthorized entry. The cave remains closed to visitation.