They use miners in WW1 to dig under the enemy to plant explosives. Trouble is the enemy did the same. Sometimes they could hear each other passing by. Officially a sapper is a soldier who lays, detects or disarms mines but became the common name for a Royal Engineer. Going back to mines dropped from aircraft, they were usually attached to a parachute otherwise they were likely to detonate when hitting the ground, then it would be a bomb.