Which cave system is the longest in the United Kingdom?

The Three Counties System is the longest cave system in the United Kingdom. It is an interconnected network of limestone caves beneath parts of Cumbria, Lancashire and North Yorkshire in northern England, with roughly 90 kilometres of surveyed passage.

The system includes major cave areas such as Ease Gill Caverns, Ireby Fell Cavern, Notts Pot, Lost Johns' Cave and Pippikin Pot. These were explored as separate systems for decades, but cavers suspected they were connected underground. The idea of a larger super-system was proposed by Dave Brook in 1968.

Important breakthroughs gradually joined the sections. Rift Pot connected with Ireby Fell Cavern in 2010, while the 2011 connection between Notts II and Lost Johns' Cave linked the northern and southern portions. Those discoveries transformed a collection of neighbouring cave systems into the Three Counties System recognised today.

The name refers to the three historic counties spanning the network, not to three individual caves. The system is also deep—about 253 metres—and contains underwater passages explored by cave divers. Ogof Ffynnon Ddu is deeper in the UK, but it is not the country’s longest cave system.