204 men and boys went underground at Hartley. The pit had only one shaft — and one way out. New Hartley, Northumberland, 16 January 1862. The day shift was already at the coalface when the colliery’s huge cast-iron pumping beam — over forty tons of it — snapped without warning and crashed down the shaft. It tore through the lining and jammed solid. The only route in or out was sealed. The falling beam did not kill the men below. The blocked shaft did. With nothing to move the air, the pit slowly filled with foul gas while the whole village waited at the surface in the January cold. Rescuers dug for almost a week. When they finally broke through, all 204 were dead — suffocated in the dark. The youngest was ten. Everyone knew a single-shaft pit had no second way out. But one shaft was cheaper to sink. It was only after Hartley that the law was changed to require two shafts in every British mine — the law those men earned and never got to use. When a disaster is built into the design,...