The Devil's Appendix () is the tallest single-drop waterfall in Wales and one of the tallest in the United Kingdom. It is a plunge style waterfall located on the Clogwyn y Geifr cliffs beside Twll Du in Cwm Idwal, Snowdonia, Wales.
It is formed where a small stream falls for approximately to reach the slopes above Llyn Idwal.Paul Williams, Rock Climbing in Snowdonia, Frances Lincoln, 2006 Depending on flow and the ambient temperature, it can be either an ice climbing route, a single drop waterfall, or a broken waterfall.
See also List of waterfalls List of waterfalls in Wales
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