Four workers were injured Friday in an explosion at a disused quarry here, police and firefighters said.
Three of the injured workers were trapped briefly in the quarry and were treated for hypothermia. The fourth, who was outside the quarry when the blast occurred, suffered bruising to the head after being hit by a stone, they said.
The explosion took place at around 10:10 a.m. after one of the workers in the quarry used a lighter to burn a piece of string binding some tools, they said.
They added that a gas buildup of some kind had occurred in the pit, where a rock known as "oyaishi" was quarried.
With the site located some 5 km northwest of the center of Utsunomiya, the blast damaged the roofs of nearby houses and hothouses.