Grant and Meade suddenly decided to use the mine three days after it was complete after a failed attack known later as the First Battle of Deep Bottom. Union soldiers filled the mine with 320 kegs of gunpowder, totaling 8,000 pounds (3,600 kg). The explosives were approximately 20 feet (6 m) under the Confederate works, and the T-gap was packed shut with 11 feet (3 m) of earth in the side galleries.