The workers were welding iron bars 15 meters (49 feet) underground when the explosion occurred in the morning, said officials from the Gyeonggi Province Fire and Disaster Headquarters, who didn't want to be named, citing office rules.
One worker was found dead above ground, his body possibly blown upward by the force of the explosion, while three others were found dead underground. Three of the injured were seriously hurt, the officials said.
The cause of the explosion wasn't immediately known. An official from the Namyangju fire department, who also didn't want to be named, citing office rules, said a gas tank used for the welding operations might have exploded.
There was public outrage about the death of a 19-year-old subway worker who was hit by a subway train on Saturday while doing maintenance work on screen doors on a station platform in Seoul. Critics have questioned the labour policies at Seoul Metro, the subway operator, asking why the man was working alone when safety regulations require at least two for such jobs.