In 1905 the Ebbw Vale Urban District Council initiated digging of the tunnel to try and penetrate Mynydd Llangynidr and intercept the underground water flow to Ffynnon Cae Rhos, a large spring in Cwm Claisfer on the northside of the mountain over 3 miles away. At that time Cwm Carno only contained one reservoir at its lower end and the expanding steelworks at Ebbw Vale needed water in large quantities. The tunnel was then dug with contracts in1905,1908 and 1911 to a length of over 3000 metres. It was lined with Beaufort brick in parts where the geology demanded it and was floored with concrete containing a drainage channel over the majority of its length.
However, in 1911 the diggers hit problems in the form of an extensive shale band which the tunnel penetrated and which continually collapsed making the workings difficult and dangerous. The work thus becoming difficult, a consultant geologist from London, H. Lapwotth DSc.FGS. was called in to inspect the tunnel and the surrounding area from a geological standpoint and pronounce on whether continued digging would be likely to produce the volume of water which was needed. His report in 1912, which is lodged at present in the County Records Office for Gwent in Cwnbran I summerise below and should be read alongwith reference to the survey of the tunnel which I and members of Chelsea Speleological Society carried out in the spring of 1982.