Short answers, pulled from the story.
Drift mining is the extraction of ore or coal through near-horizontal tunnels driven directly into a hillside, following the seam or vein. Unlike shaft mining, which requires vertical excavation and machinery to pump water and raise coal, drift mines enter above the water table and rely on gravity for drainage, costing less than half the amount required for shaft operations.
Drift mining was practiced across the British Isles from at least the 13th century. Records from that period document coal digging in Durham, Northumberland, Nottinghamshire, Derbyshire, Staffordshire, Lancashire, and Gloucestershire in England; Lothian in Scotland; and North and South Wales. Aberpergwm in South Wales remains the largest operating drift mine today.
Nome drift miners sank prospect shafts by building fires to melt the permafrost and shoveling away mud, repeating the process down to bedrock. When gold was found, they tunneled horizontally along the bedrock surface to follow pay streaks. The frozen ground kept tunnels stable without timbering. Around 1900, more than twenty thousand people lived in Nome, many working these underground passages.
Eastern Kentucky drift mines face roof collapse caused by hillseams, weather-enlarged tension joints in shallow overburden where surface slopes are steep. A 1989 U.S. Bureau of Mines study identified hillseams as the dominant geologic cause of roof instability in the outcrop barrier zone, with many injuries and fatalities attributed to them. The danger is greatest within 100 feet of the mine portal.
After Judge Lorenzo Sawyer's 1884 decree banned hydraulic mining debris from the Sacramento River, drift mining of gold in buried Tertiary channels partially offset the resulting production loss for roughly fourteen years. Overall production still declined during this period. The first successful gold dredge, introduced on the lower Feather River near Oroville in 1898, ultimately replaced drift mining as the dominant method.
Andrew Roy's 1880s report noted that drift mines generally hold advantages for loading coal rapidly over shaft mines. In Ohio's Hocking valley, large drift mines could produce 1,200 to 1,500 tons per day, compared to 600 to 700 tons for shaft mines. Roy also documented that opening a shaft mine one hundred feet deep could cost upwards of $20,000, with some cases reaching $100,000, while drift mines cost less than half that amount.