The failure of this enterprise ended operations in that quarter with such purpose for several weeks to come. Another plan was formed by which to break Confederate resistance. Immediately in front of Petersburg the two opposing lines of fortifications lay at one point within less than fifty yards of each other. Each line was strongly built and each was protected at every point by traverses,--earthworks built at right angles to the main works, as a protection against an enfilade or cross fire. So close were the works together, and so incessant was the fire that it became at last impossible for men on either side to show their heads above the works in order to discharge their guns. On either side port holes were made by the placing of sand bags on top of the parapets, in such fashion as to leave holes through which the men might fire their guns. Even these port holes were unavailable for use if by any chance the enemy looking toward them through a port hole on the other side could see the sky beyond. The moment a man undertook to shoot through the port hole his head, obscuring the light, revealed his presence there to some sharpshooter on the other side who was standing ready with gun aimed and "bead drawn" waiting to fire into the hole the instant the sky beyond should be obscured by human presence. So ceaseless was the fire at this point that repeated experiments showed that any twig thrust above the crest of the parapet would be instantly cut in two by one of the multitudinous bullets which were flowing in a continuous stream from one side to the other. The space between the works was so perfectly and completely commanded by Confederate artillery that no general in his senses would have thought of attempting to cross it, even with the most heroic of veterans. But just in rear of the Federal lines there was a cavernous hollow between the hills, where anything might be done without the possibility of Confederate discovery. A regiment composed mainly of Pennsylvania coal miners was brought to that point, and instructed to push a mining shaft under the hill in order to plant a great mine immediately beneath the Confederate works. The tunnel began in the ravine in rear of the Federal works, and extended thence 500 feet. This brought it immediately under an important redan in the Confederate lines. There a cross gallery eighty feet long was dug, and packed full of gunpowder,--8,000 pounds in all. The plan was to surprise the Confederates and break their lines by the explosion of this tremendous mine on the morning of the thirtieth of July. It was intended to take advantage of the confusion thus created, and push a strong column through the gap made in the works, thus cutting Lee's army in two, and compelling it to retreat.