|U.S. Marines with Maritime Raid Force, 26th Marine Expeditionary Unit, breach a door with a demolitions charge during breach training at Fort Pickett, Va., April 10, 2015. U.S. Marine Corps photo by Cpl. Joshua W. Brown.|
Anyway, we don't see those signs anymore because about 25 years about Nonel replaced electrical detonators and now most everyone uses it. Nonel is a plastic tube filled with a low grade explosive. You trigger it at one end and the explosion races down the tube and triggers the blasting cap at the other end. This explosion causes the tube to light up, but only for a split second, and somehow the photographer managed to capture it here.
The brief flash of light you see before the ground erupts is the network of Nonel strung to all the explosives buried in this blasting site. Dyno Nobel has a video up on YouTube that gives a good overview.