Seven miners were killed on Monday when they were poisoned by carbon monoxide at a small private coal mine in Hequ, a county on the Yellow River inside Shanxi Province, according to a local source.
The accident took place at 10:50 a.m. at Xinwang Coal Mine when 10 miners who were working inside the shaft broke into a deserted area. This caused a sudden leak of carbon monoxide, the Shanxi Provincial Bureau of Work Safety said in a statement.

Only three of the 10 miners made their way back to the surface.

The cause of the accident was under investigation.

Dubbed a "sea of coal", Shanxi is the country's largest coal producing area with a reserve of more than 260 billion tons. It produces 580 million tons annually.