Although referred to as caves (and marked as such on OS maps), these are actually mines, presumably an offshoot of the main 19th century lead mining area at nearby Talargoch. The mines at Glan Llyn consist of a series of deep and impressive chambers cut through the rock at the base of ravine, the entrance to which is partly blocked by rubbish and other debris. There are high-level and low-level workings, connecting to a sizeable network of underground passages. The walls bear the marks of drill holes throughout, proving them to be man-made and not natural caves.
Source: Paul Davis April 2018.