Plumbing the waterworks

By Martyn Farr

Cave divers recently descended a wellshaft owned by Welsh Water and explored a network of caves thought to be the biggest this side of the Yorkshire Dales. They found tree roots, horse bones (possibly ancient) and more shafts awaiting investigation. Opposite: Pat Cronin prepares to negotiate the space between the water board's extraction pipes at Schwyll, also known as The Great Spring of Glamorgan



WE scrambled down a couple of short steel ladders and dodged around a few other heavy fixtures. Seconds later we were at the bottom. From the narrow metal platform the view was interesting - most definitely different.
Just in front of us and beneath our feet, four huge iron pipes led into the water. It was a chilly spot, but the place hummed and vibrated like the inside of a large ship. I gazed around, trying to take it all in.
"We'll have to turn those off before you get in there," said Alan, the site supervisor.
We were in the well shaft of a major pumping and filtration plant run by Welsh Water, and there was no doubt in my mind that down below would not be a safe spot to be if water was being extracted.
"We take five million gallons a day from here with no problem," said Alan. "In the drought of 1976 we took eight million gallons!"
A tremendous volume of water was powered into the chamber from a low-roofed alcove off to the right, and to all intents and purposes it was pur,e clean water. To a diver normally accustomed to visibility of less than 2m, this place was a gem. The visibility here was in excess of 6m!
"It's nearly always like this; it's about two or three days after any heavy rain before the water seems to discolour. It comes from a long way away," said Alan.
I had read the limited sources of information about the place, and remembered the hydrology map that had once adorned an upstairs room in the main filtration building. Like a segment of a spider's web, dotted lines trailed away from this spot, presenting a mind-boggling concept - the water emerging from this set of springs came from a distance of more than 20 miles!
I had found this hard to believe when I had first seen it, but clearly the people at Welsh Water still give credence to the idea. Certainly, from the water characteristics and the delayed response after heavy rain, the supply that they tap here must have travelled a great distance to reach the spring.
This place is Schwyll (pronounced Shwill), known in historical times as the "Great Spring of Glamorgan". Where the name comes from or what it means is as much a mystery as the source of the water itself.
Ask virtually anyone about the place and they will look at you blankly. Divers have certainly never heard of it, as it is a freshwater rising percolating to the surface via a series of narrow, blocked fissures in the banks of the Ewenny River. Few cavers have ever heard of it either, as there is no known cave passage associated with the underground waterway.
To put the place into some perspective, consider that the volume of outflow here is greater than that from Wookey Hole or Cheddar Risings in Somerset, and it is certainly far larger than any other freshwater spring in Wales. Here at Ogmore, near Bridgend in South Wales, we have one of the largest risings in the British Isles, and it is virtually unknown.
I had heard of Schwyll back in 1971 when I first started cave diving. In February 1974, on a clandestine outing, I had attempted to gain entry to the hypothesised labyrinth by sneaking into the small opening hidden behind a wall on the opposite side of the main trunk road.
Equipped with a single cylinder, no watch, compass or depth gauge, I had wriggled a few metres into a tight fissure before it became impossibly tight and utterly impassable.
It was obvious that there was no accessible cave/sump at Schwyll, unless entry could be gained via the pumping shaft in the main compound of the treatment works. Back in the early 1970s, there seemed little prospect of cave divers being allowed to explore this shaft, and that was the situation for many years.
In the summer of last year it was learned that the Schwyll plant was off-line, and that Welsh Water was in the process of re-evaluating the site, so no abstraction was taking place. Through a series of telephone calls I managed to arrange some insurance and access to the site.

Right: the cave system at Schwyll. The outflow here is greater than that from Wookey Hole or Cheddar Risings, in Somerset, and far larger than any other freshwater spring in Wales. There is little doubt that the underwater cave network is the most extensive south of the Yorkshire Dales.

After more than 23 years I was back, scrutinising the shadowy depths of the main well shaft. It was 5 November, 1997 - this plot of mine had certainly taken its time to hatch!
Over the summer, I had read something in a caving publication that had got my mind racing - it seemed that anyone who dived here would be in for quite an adventure:
"Employees working at Schwyll report that during an exceptionally dry period, a group walked from the well up a passage for a considerable distance only to be stopped by a tight continuation.
"I visited the well during the exceptional drought of 1984 and discovered a rock alcove that gave access to a rift about 4m high, of which 3m was flooded. Access could have been gained by swimming along a half-metre wide passage... and (it) appeared much wider in its flooded section.
"If access were allowed at Schwyll... it is possible that the flooded section would give access to an upper dry fossil system of considerable size. Perhaps after a few millennia, when the water level drops, there will be one hell of a through trip from South Powys to Schwyll."
However, site supervisor Alan was doubtful about how far we would get: "We've had divers in here and they said it was too small; the water seems to come out of some small cracks," he said.
Down at the water's edge, spectators began to fill the limited area of the gantries. The current was strong but I paddled over to the upstream side of the pipes and began to look around. Very little, if anything, seemed to correspond with the published accounts. Conversely, there were small fissures, and the water welled up them under great pressure.
We had two possibilities for making headway. In the alcove at surface level there was a small flooded tunnel, less than half a metre wide, that we thought might be passable after digging.
However, just a metre or two away from this, there was an obvious fissure leading steeply down behind one of the large pipes. This was to be the key.
Once kitted up I wriggled past the pipe and found that there was a narrow, restricted cleft dropping vertically away for an unknown distance.
I returned to the surface and relayed the finding before advising the gathering of the possible dive duration - with a substantial air supply I could be away for an hour or so. A distinct murmur of concern echoed from above.
Clearing loose rocks as I went, I dropped slowly down the rift to land on a pile of bricks and debris at a depth of 7m. From this point a small but comfortable-sized tunnel meandered quietly on down. I reached 9m depth and was fully expecting to continue much deeper, but with the superb visibility, the passage could be seen to ascend. A further 20m along I was in altogether more shallow terrain, at a depth of less than 3m. A basic knowledge of local geology and hydrology told me that the water came from the north. Logic therefore demanded that I consult my compass to try and maintain this heading.
At the first junction I checked orientation and veered left. Just beyond, dense clumps of cream-coloured tree roots rippled like fronds of seaweed in the current. This really was shallow water now, and I looked up to see the tell-tale mirror-like silvery surface of an airspace.
I tried to push my head into the space but to no avail. The few centimetres of air lying between the water and the rock above were a cruel deception. I moved on northward. Another airspace appeared and this time I could get my head and shoulders out of the water, but that was all. It was an isolated pocket of air contained within cold damp walls and set beneath a matrix of mud and cobbles above.
I tied the line off to mark the spot and carried on, but I soon realised that the water was travelling the same way I was. Evidently I must have turned into another outward-trending tunnel, one that would eventually lead to one of the other springs in, or close to, the Ewenny River.
The line was cut, and I turned around to make my way back to the first junction to locate the origin of the flow.
At the junction, the route continued in a predominantly easterly direction. Considerable time was spent securing the line along this main tunnel, but given the very shallow depth, little air was consumed.

Approximately 150m from the dive base, we reached an area of bones, like something from a large prehistoric creature. I placed a blackened leg bone to one side to remove on exit. A further 20-30m of line was then laid along a similar trend before I noted that I had been underwater for 36 minutes.
It was essential to give those on the surface no cause for concern, so I floated quietly back down the line to surface after 50 minutes away. It had proved a most amazing day.
On the second dive the line was extended to a point 370m from base, and on the third push the diver reached 440m. At 400m a large cavern was gained with a substantial air surface - the stage was set for a very productive exploration beyond. Apart from the bones (since identified as a species of horse) which await dating, the really intriguing discovery concerned a series of shafts leading to an even lower series of tunnels.
In a British context, there is now little doubt that Schwyll is the most extensive underwater cave network south of the Yorkshire Dales. Given the geological structure of the area, it is interesting to speculate that the waterways continue for a considerable distance.
Cowbridge lies 6.5 miles to the east, and at some time in the future a route between the two points could well be achieved. Also, there are numerous reports of ground collapse in the vicinity of Bridgend, and certainly hydrological work gives clear evidence of a direct connection from Coychurch four miles to the north.
To the west, passages may well continue out beneath the Ewenny and Ogmore rivers, and possibly even 2 miles or more in a southerly direction towards the sea. And of course the most intriguing question of all is whether or not some water does indeed flow to Schwyll from the Brecon Beacons 20 or so miles away, or even from 30 miles distant, from Pontypool!

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Appeared in DIVER - February 1998

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