This is an old miners' expression for successfully returning to the surface, particularly during an emergency evacuation or at the end of a gruelling shift. The phrase contrasts the dark, sterile rock of the underground workings with the green vegetation of the surface world. It is still used informally to describe escaping a dangerous underground situation.
Heard on the job
When we smelled smoke in the return airway, the shift boss told everyone to drop their tools and get to grass immediately.