The Hölloch is a 200.4 km (124.5 mi) long cave in the Muotathal municipality in Switzerland. It is also notable for having a depth of 938.6 m (3,079 ft) and being the second longest cave in Europe. The initial exploration started in 1875 and was led by Alois Ulrich. A large part of the exploration of this cave was led by one of the pioneers of speleology, Alfred Bögli. The explored length of the cave increased from 25 km (16 mi) in 1952 to 100 km (62 mi) in 1968 (it was the first cave in the world where the explored length reached 100 km). Until September 9, 1972, it was once considered to be the largest cave complex in the world. A Cave Research Foundation mapping team led by Dr. John P. Wilcox, Patricia Crowther, Richard B. Zopf, Dr. P. Gary Eller, Stephen G. Wells, and Cleveland F. Pinnix (a National Park Service Ranger) managed to pursue a low, wet passage that linked two of the area's long cave systems—Flint Ridge Cave System to Mammoth Cave. This connection made the combined Flint–Mammoth Cave System the world's longest cave, surpassing Holloch.