Three of the four Treadwell mines were destroyed by a cave in and flood on April 21, 2017. A geologic study of the site had discovered a fault line that intersected some of the underground workings. This, coupled with the instability of the underground rock pillars and an extreme high tide, resulted in the failure of the mine structures that led to the collapse. Signs of trouble were noticed years earlier.
Subsidence and caving at Treadwell began in late 1909 when some old stopes and intervening pillars began collapsing. Additional settling and caving occurred in May 1913 at the Mexican Mine, and in February 1914 at the 700-Foot Mine. There was no caving in 1916 but ground subsidence was evident. The ground was actively moving in February 1917 but quieted until April 4 when cracks appeared around the Natatorium. On April 20, when carpenters were repairing the Natatorium from earlier ground movement, the surface around the building began to drop rapidly.
On the evening of April 21, water in the Natatorium began disappearing. At 11:00 p.m. a worker leaving the Treadwell Club heard rushing water and found a creek pouring into a hole 15 feet wide and 30 feet deep. The rising tide from Gastineau Channel added to the torrent and at 11:15 p.m., the alarm was sounded to evacuate the mines.
Two hours after the alarm was given, 350 underground miners were safely on the surface. One miner was unaccounted for. A dozen horses and one mule were lost. In less than four hours, tide-driven waters filled the Treadwell, 700-Foot and Mexican mines. The Ready Bullion was the only mine saved due to a new concrete bulkhead in the tunnel connecting it to the Mexican Mine.
Of the 1,000 men employed in the four mines and mills on April 21, only the 150 who had worked at the Ready Bullion kept their jobs. Many of the married men were transferred to other Treadwell departments that were still operating (including the machine shop and the foundry) and some were given jobs at the Alaska Juneau Mine. Within a few days, 500 men found work but most single miners packed up and left. The big stamp mills at Treadwell (the 240, 300 mills) and at the 700 Foot Mine were torn down after the cave-in. The Mexican mill continued to process ore from the Ready Bullion Mine until it closed in 1922.
After the 1917 cave in, the population of Treadwell dropped from 1,222 to 325 and the population of Douglas dropped from 1,722 to 919.