The Easegill Cave System is the longest cave system in the British Isles, with over 100 km (60 mi) of passages, including connections only passable by cave diving.

The first entrance to be discovered, Lancaster Hole, was found by George Cornes and Bill Taylor on 29th September 1946. A small draughting opening on Casterton Fell, Lancashire, opened immediately onto a 35 m (110 ft) shaft. Passages from the base of the shaft were explored over the succeeding weeks and months by members of the British Speleological Association. The underground course of the Easegill beck (the local master cave) and high-level fossil passages above it were found and followed upstream to a series of complex inlet passages. In succeeding years these have been connected to surface caves, including Top Sinks, County Pot and Pool Sink.

The cave passages adjoining the Easegill main streamway were connected to Link Pot and Pippikin Pot in 1978, and Pippikin was itself connected to Lost John's Cave by diving in 1989.

With its many entrances, the Easegill system offers cavers a wide variety of through trips; the Easegill streamway is regarded as one of the finest in the UK.