Yes you are correct there is   waterfall in Wales that is famous as you can walk behind called Sgwd yr Eira (“Fall of Snow”).

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Location: In the Brecon Beacons National Park a photographers dream (Waterfall Country, near Ystradfellte).

A natural footpath runs behind the curtain of water, so you can literally walk along the rock ledge and stand behind the waterfall. Be careful as it does get wet and slippery. 

Around a 4-mile (6 km) walk from the nearest car park, through beautiful woodland and riverside trails.

It’s one of the most popular waterfalls in Wales for exactly this reason.

It is one of our favourite Waterfall Walks. What makes it special is it is a Waterfall in Wales you can walk behind and experience the power of the Water as it drops from above you.  Definitely one to add to your list to explore.