These are one of the falls in Waterfall Country that you can walk behind and also the tallest in South Wales - 90ft! It sits  on the western edge of the Brecon Beacons nestled within  Graig Llech Woods.

The falls are best seen after a heavy rainfall but please be careful as this does make the paths very slippery.

Some information taken from the National Trust site:

The falls occur on a geological fault on the river Nant Llech and have retreated up the valley by up to 165ft (50m) since the last Ice Age. The hard layer of sandstone forming the lip of the waterfall is known as Farewell Rock. Coal miners digging down to this layer in the mines of the South Wales valleys would say 'farewell' as there was little chance of finding coal below it.

In the mid-1800s the area was surveyed by William Logan, an internationally noted geologist. While surveying for a detailed geological map of the South Wales coalfields he found two fossilised trees at the base of the falls. You can now see them outside Swansea museum. 

So, what you may not know (although it is isn't a very well kept secret), is that the entrance to Batman's cave is actually in the Brecon Beacons! Who would have thought it, eh?