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Ogof Ffynnon Ddu

Ogof Ffynnon Ddu (Welsh for cave of the black spring), also known informally as OFD, is a cave under a hillside in the area surrounding Penwyllt in the Upper Swansea Valley in South Wales. It is the second longest cave in Wales and the deepest in the United Kingdom.

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Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).

More caves in United Kingdom

  1. Mossdale Caverns76 m deep
  2. Dan yr Ogof17 km
  3. Swildon's Hole9.1 km
  4. St Cuthbert's Swallet7.1 km
  5. Wookey Hole Caves3.7 km

Other notable caves worldwide

  1. Mammoth Cave · Australia686 km
  2. Sistema Sac Actun · Mexico119.2 m
  3. Lechuguilla Cave · United States489 m
  4. Optymistychna Cave · Ukraine20 m
  5. Veryovkina Cave · Georgia2,212 m
  6. Krubera Cave · Georgia2,199 m
  7. Sarma cave · Georgia1,830 m
  8. Clearwater Cave · Malaysia109 km