Fyfield is charged in the deaths of four workers — David Powell, Charles Breslin, Philip Hill and Gary Jenkins — at the Gleision drift mine near Cilybebyll in the Swansea Valley after it flooded in September 2011.

Investigators estimated that within four minutes, 500,000 gallons of water engulfed the section of the shaft where the men were working after they penetrated old flooded workings.

The company, which owned and operated the project, MNS Mining Ltd., also faces four charges of corporate manslaughter; company officials similarly deny wrongdoing. The proceedings are expected to last six months.