Observing that the closest Confederate fortification, Elliott's Salient, was a mere 400 feet from their position, the men of the 48th conjectured that a mine could be run from their lines under the enemy earthworks. Once complete, this mine could be packed with enough explosives to open a hole in the Confederate lines.
This idea was seized upon by their commanding officer Lieutenant Colonel Henry Pleasants. A mining engineer by trade, Pleasants approached Burnside with the plan arguing that the explosion would take the Confederates by surprise and would allow Union troops to rush in to take the city.
Eager to restore his reputation after his defeat at the Battle of Fredericksburg, Burnside agreed to present it to Grant and Major General George G. Meade. Though both men were skeptical about its chances for success, they approved it with the thought that it would keep the men busy during the siege. On June 25, Pleasants' men, working with improvised tools, began digging the mine shaft. Digging continuously, the shaft reached 511 feet by July 17. During this time, the Confederates became suspicious when they heard the faint sound of digging. Sinking countermines, they came close to locating the 48th's shaft.
Having stretched the shaft under Elliott's Salient, the miners began digging a 75-foot lateral tunnel that paralleled the earthworks above. Completed on July 23, the mine was filled with 8,000 pounds of black powder four days later. As the miners were working, Burnside had been developing his attack plan. Selecting Brigadier General Edward Ferrero's division of United States Colored Troops to lead the assault, Burnside had them drilled in the use of ladders and instructed them to move along the sides of the crater to secure the breach in the Confederate lines.