The Easegill System in the Pennines, is with over 60 miles of known passage, the longest cave system in the British Isles. It is also known as the Three Counties System since it straddles the borders of the three counties of Yorkshire, Cumbria and Lancashire.
After many years of exploration and water-tracing studies, sometimes employing mining and explosives to gain access to narrow passages and clear rockfalls, and cave-diving to explore submerged passages, cavers have connected several of the largest cave systems in the Yorkshire Dales, on the flanks of Gregareth Hill, into a single system. These include Aygill Caverns, Bullpot of the Witches, Easegill Caverns, Pippikin Pot, the Leck Fell system and Long John's.
Although a few people have been killed by rockfalls within the caves, and others occasionally lost or trapped by floods, statistically cavers here are noticeably safer than walkers on the surface above.