A crack you could slip a penknife into. Hours later it killed 240 people in their sleep. In 1864 the brand-new Dale Dyke Dam stood full for the first time above the Loxley Valley near Sheffield. A workman spotted a crack in the embankment and reported it; the chief engineer rode eight miles to inspect it — by then wide enough for a hand — and judged the dam would hold. Just before midnight it gave way. 650 million gallons swept down the valley. In one cottage a labourer got his wife and six children onto the roof; the water took all eight. The inquiry blamed a landslip. No one was punished. The warning had been seen in time. #BritishHistory #ForgottenHistory #Sheffield