Malcolm Fyfield maintains his innocence, despite facing four counts of gross negligence manslaughter.
Fyfield is charged in the deaths of David Powell, Charles Breslin, Philip Hill and Gary Jenkins who died at the Gleision drift mine, near Cilybebyll in the Swansea Valley, after it flooded in September 2011.
Detectives assessed that within four minutes, 500,000 gallons of water submerged the section of the shaft where the men were working after they breached old flooded workings.
MNS Mining, which operated the project, also faces four charges of corporate manslaughter.
Company executives similarly deny any wrongdoing.
Proceedings are expected to last six months.
