The Dan-yr-Ogof Showcaves were discovered 100 years ago today by local farmers, in the hills of the Upper Swansea Valley. As James Wright finds, they offer a glimpse much deeper into Wales' geological past.
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Rain overnight turning light and patchy in the morning.
Most of us will have a pleasant and dry evening with a few late sunny spells, though the odd light shower is possible.
The Eisteddfod's intention to visit an area must be proclaimed at least a year and a day before the beginning of the festival.