Well no mystery really, just geology but I'm sure the early inhabitants of the land had quite a different view of things.
This walk starts at Pont Melin Fach in South Wales with a scramble up Afon Nedd. It starts gently through woodland along the side of the stream but rapidly becomes more challenging as it follows the stream up the valley. The scenery is lovely, the overhanging trees make a damp micro-climatic in which ferns and mosses thrive. The downside is that the path can be wet and muddy and the rocks slippery.
As you can see this is a fair size river at the point - however all of a sudden it just disappears - actually it is reappearing rather than disappearing! Photos really don't show this well so a short video is in order I think. I'm sorry about the quality. Being balanced on wet mossy rocks doesn't help a steady hand! You can see it bubbling up though nicely.
Another re-appearance; this is Pwll Du. The tributory stream flows out of the cave in the rock but the water itself seems to barely move.
In both cases the water has run into a sink hole or swallet hole much higher up the valley and made its way underground until it reappears much lower. The overall effect though is quite surreal with the sound of the river which is considerable just disappearing as you climb the valley.
After this the actual disappearance of the river when you get to it higher up is a bit of an anti climax.
After following the river a little futher the path wends its way over the top of the mountain through a strange almost lunar landscape.
These are shake holes- depressions in the ground caused by the underground rivers dissolving and washing away the rock. The ground above collapses leaving these round depressions looking like someone has taken big scoops out of the ground with a giant spoon.
Having seen no one all morning, the adjoining valley of Afon Mellte seems very busy. This is much more accessible with a car park nearby so many more visitors.
Here the river Mellte disappears into a cave. It is popular with cavers and pot holers. The river seems to just stop in a pool with water flowing in endlessly but the pool with no apparent visible exit gets no bigger. You can hear the rush of the ( now invisible river from the back of the cavern but it isn't accessible without the proper caving equipment.
The rush of the water can be seen here though. Again photos don't give any idea of the speed and noise of the now subterranean river so another short video.
Nearly back now past the ( reappeared) river and past some spectacular waterfalls. Here is just one of them.
All in all a really lovely day out even without a pub for lunch - now if we hadn't got slightly lost we would have been able to make a short detour to the nearest village - oh well there is always next time!