Nant Y Gro Dam is located a short distance upstream of caban Coch Dam. Map Ref SN921 634
The small masonry dam was built across the Nant-y-Gro stream in the early stages of the construction of dams in the Elan Valley. This dam created a 500,000 gallon reservoir which was used to provide a water supply fed by a pipeline to the navvies village below Caban Coch Dam. The dam was capable of suppling up to 45,000 gallons of water per day. The government requested the use of the 35 feet high dam for secret experiments during World War II. Barnes Wallis who invented the Bouncing Bomb carried out top secret experimental trials on Nant y Gro Dam. In May 1942 the first live explosive tests on the Nant-y-Gro dam were carried out, watched by Barnes Wallis. These first attempts were spectacular, but they did not seriously damage the Nant-y-Gro dam, another attempt was made in July 1942. A mine was suspended at the optimum depth by scaffolding from the mid point of the dam, and detonated remotely. A huge central section of the dam wall was successfully blasted away in a massive explosion. The Bouncy Bomb wasn't itself used in the Elan Valley, it was successfully use in breaching a series of large dams in the Ruhr Valley in Germany.