Things that go whoosh in the night

Last year I was on about Aluminium: how we depend upon it to hold 7-Up and Indian take-aways and 500 people aloft in a wide-body aeroplane.  Aluminium the element is abundant in the lithosphere but aluminium the metal is extremely costly to produce.  The key process is electro-smelting of aluminium oxide Al₂O₃ which requires megawatts of electricity. That is why aluminium plants are usually situated near a source of hydro-electric power.  More than 100 years ago in 1907, the Aluminium Corporation of Dolgarrog started to output tons of aluminium sheets. This venture depended on damming a tributary of the Conwy River in North Wales and running a pipe [R with adjacent tramway] from the Coedty Reservoir formed behind the dam [290m elevation] to the turbines of the hydro-power plant below [50m elevation].
In the 1920s, the rapacious directors decided to dam Llyn Eigiau a further 100m higher up the valley.  That dam was poorly surveyed, poorly engineered and poorly built with sub-standard materials on superficial foundations. On the night of 2nd November 1925, after a week of torrential rain - 650mm in five days! - the Eigiau dam failed and released a 15m wall of water which over-topped and then destroyed the Coedty embankment below and carried on downhill to wipe out the lower part of the village of Dolgarrog.  The power of 1.5 million tons of water was able to shift 500 ton boulders, so the puny works of man didn't resist for long. The church and a couple of rows of houses were batted aside and at least 16 accounted people were killed. With a fair amount of turn-over in the works work-force nobody can be sure if there weren't itinerant labourers billeted in the village who disappeared without trace. Luckily, it was film night at the village hall, so much of the population was above the plume of destruction and survived.  It was quite similar to the Vajont dam burst 40 years later.  In that case I calculated that it took 25,000 tons of water to kill each of  2000 people.  The Dolgarrog disaster was much smaller both in absolute, but also in relative, terms: it took 100,000 tons each to kill the misfortunate Welsh people. Dams are one of the mightiest engineering feats of which man is capable (beavers Castor canadensis are pretty good too) but they have to be built right and in sensible positions or they are a ticking time-bomb that is difficult to defuse or deconstruct.