Torwood Castle stands about two miles north east of the town of Denny. It's abandoned but well known. A little less well known, and largely hidden by forest, is the nearby Tappoch Broch, a rare example of a type of structure much more common in the Highlands and Islands. Not far from the castle and the broch, and much less well-known than either (except, apparently, to people living locally who have swum in it for generations), is Torwood Blue Pool, a mysterious man-made circular brick structure that is full of remarkably blue water.
There are no signposts to the blue pool, so the first challenge is finding it. A gated and well-established path that has in its past seen considerable maintenance in terms of a wall on one side and a fence on the other, runs south west from the track a couple of hundred yards north west of Torwood Castle, nearly opposite the point at which a signposted path runs north into Tor Wood to Tappoch Broch.
Following this path for a little over half a mile along the edge of Tor Wood brings you to the first of two lines of high voltage electricity cables, suspended between large pylons. These cables roughly from the south east to the north west, and the forest has been cleared (or, more accurately, was never planted), for some distance either side of the line of cables. An informal beaten path leads from the path you have been following up the right-hand side of this gap in the forest, and after a hundred yards or so this brings you to the blue pool, sheltering half under the trees on the side of the gap. (Continues below image...)
The pool didn't look at its most pristine on the day we visited. Water we've seen described as crystal clear was fairly smoky in appearance. The surface was also covered in pine needles and larger debris from the trees. It was, however, indisputably blue: and very mysterious.
The Torwood Blue Pool is some 20ft in diameter and more than 15ft deep. The beautifully circular wall is three bricks thick, and has an external lining of waterproof clay. We could see no sign of it, but there is a brick lined arch below water level on one side that is the start of a brick tunnel running slightly downhill for around 30ft. A few other fixtures and fittings in terms of pipes and small outlets have also been discovered.
Over the years there has been no shortage of theories about the purpose and origin of the blue pool. What we do know is almost entirely the work of a local man, the late Nigel Turnbull, who died in 2012. His website torwoodbluepool.co.uk is maintained in his memory and continues to be an online meeting place for all those with an interest in the pool.
There remain a number of active theories about the origin of the shaft that contains the pool. For us, the most convincing explanation revolves around the presence, a mile or so to the north west, of a spoil heap that is all that remains of Quarter Colliery. Once part of a much larger coalfield, this was initially sunk in 1865 to mine ironstone, before turning to coal production in 1880. In 1895 thirteen miners were killed in a explosion. The colliery shut down in 1910 and subsequently flooded. It seems highly likely that the blue pool is what is left of a colliery air shaft intended to allow the workings Quarter Colliery to be ventilated. Remote air shafts for collieries were often constructed to a "dog-leg" design that involved a short vertical shaft descending from the surface, linked to a horizontal tunnel that then connected to a much longer vertical shaft descending to the mine workings. The purpose of the "dog-leg" was to avoid anything being thrown into the top of the shaft dropping down to the mine workings and causing damage or injury.
To fit with the normal pattern of shafts like this, the brickwork would initially have extended some distance above ground level, again to help avoid objects being thrown or people falling into the shaft. This ties in with a story from one local resident of there being a five-foot high brick wall around the shaft in 1954; and Nigel Turnbull's own memories from his first visit in 1962 of a partial wall of rather lower height. It's also worth remembering as you look around that the pylons and the forestry are much more recent additions to the landscape. One person who visited in 1983 noted there were no trees at that time. It's been pointed out that had there not been a need to leave an open strip along the line of the power cables, the shaft housing the blue pool would probably have been destroyed during the tree planting.