History.com: How the 1966 Aberfan Mine Disaster Became Elizabeth Ii's Biggest Regret

The Aberfan disaster wiped out a generation of Welsh school children and devastated the nation. An avalanche of coal waste slid down a rain-saturated mountainside. It destroyed everything it touched, eventually killing 144 people, most of them children sitting in their school classrooms. The tragedy in Aberfan would become one of the United Kingdom's worst mining disasters -- and it was completely avoidable.