The victims were doing welding work 15 meters underground when an explosion — possibly of an oxygen tank — caused the surrounding structure to cave in on top of them.
Four workers were killed, while the injured, some of them with severe burns, were rushed to nearby hospitals for treatment, a fire department spokesman told AFP.
The accident occurred around 7:20 am (2220 GMT Tuesday) in the city of Namyangju, east of Seoul,
The precise cause of the accident is still under investigation.
