The Six Companies, Inc. crew blasted tunnels to divert water around the Hoover Dam site, working in carbon monoxide-choked tunnels and with jackhammers from heights of 700 feet. Casualties resulted from falling equipment, rocks, and workers. The view includes the interior of one of the tunnels through which the Colorado River was to be diverted around the Hoover Dam site, a high scaler going over the side on the Arizona rim of Black Canyon, the completed tunnel lining at the intake portal of Diversion Tunnel No. 4 with a pressure grouting jumbo in operation, and a Hoover Dam plan from 1930.