Short answers, pulled from the story.
Open-pit mining is a surface technique that extracts rock or minerals by digging a series of stepped benches downward into the earth. Miners first drill test holes to locate and measure an ore body, then excavate on benches typically 12 to 15 metres thick, with haul roads spiraling up the pit walls to carry ore and waste rock to the surface.
Open-pit gold mining is generally viable at concentrations of 1 to 5 parts per million. At the Peak Hill mine in western New South Wales near Dubbo, Australia, a grade as low as 0.75 parts per million was made economic through bulk heap leaching.
Open-pit mining generates air pollutants from transportation, drilling, and blasting; creates toxic tailings dams containing sulfide minerals and cyanide; and can cause acid mine drainage when sulfides oxidize into sulfuric acid. By 2024, nickel mining was one of the main causes of deforestation in Indonesia, and cobalt mining has driven habitat destruction in the Democratic Republic of Congo.
Closed open-pit mines are contoured, capped with clay and soil to slow acid mine drainage, and replanted with vegetation. Some are converted to landfills. In Germany, former lignite mines have been deliberately flooded to form artificial lakes including the Lusatian Lake District, the Central German Lake District, and the Upper Palatinate Lake District.
Some waste dumps may take hundreds to thousands of years to become acid neutral and stop leaching acid and heavy metals into the environment. No long-term studies confirm the effectiveness of clay-and-soil covers because large-scale open-pit mining has not existed long enough to generate that data.
Untopping is a form of open-cast quarrying that removes overburden above a worked-out underground mine to recover minerals left as pillars. It was used in Welsh slate workings in the 1930s and the 2000s, where Martyn Williams-Ellis, manager at Llechwedd, applied newly mechanized bulk excavation techniques to keep Victorian-era workings profitable.