The idea originated with Lieutenant Colonel Henry Pleasants from Pennsylvania. He proposed to Grant that his Pennsylvania engineers dig a tunnel five hundred feet long, directly under the Confederate defenses. Explosives would be set, and a hole blasted in the enemy lines.

Grant was usually willing to entertain novel ideas, and thousands of men were soon engaged in the hot but quiet work of removing the tons of earth. That this was done without alerting the defenders suggests, yet again, that the Confederate intelligence system had been compromised. All was set for the early morning of July 30, 1864.