The collapsed mine in California’s San Diego County was a major mine with more than 7,000 workers on hand, but the collapse of the $4.6 billion project on Thursday evening sent an estimated 5,000 to 6,000 miners and their families fleeing, the Associated Press reported.
Officials at the mine in the small town of Monterey told the AP that a blast and explosion caused the collapse.
Officials say the mine’s operator was also among the workers who were injured.
The mine is owned by American Iron Ore Company, which had planned to mine ore from the Monterey Gold Rush area of California’s central valley.
The Monterey gold rush began in 1849 when a team of prospectors discovered gold in the San Joaquin Valley.
Mining operations in the area soon expanded, with gold production in the 1800s reaching $7 million a day, according to the San Jose Mercury News.
More than 4,000 gold mines were established in the region in the late 19th century, the newspaper reported.
The U.S. Geological Survey estimated the Monty ore mine at 1.7 million tons.
The Associated Press contributed to this report.