The mine was in a "T"-shape. The approach shaft was long, starting in a sunken area downhill and more than below the Confederate battery, making detection difficult. The tunnel entrance was narrow, about wide and high. At its end, a perpendicular gallery of extended in both directions. Grant and Meade suddenly decided to use the mine three days after it was completed after a failed attack known later as the First Battle of Deep Bottom. Union soldiers filled the mine with 320 kegs of gunpowder, totaling . The explosives were approximately under the Confederate works, and the T-gap was packed shut with of earth in the side galleries. A further of packed earth in the main gallery to prevent the explosion blasting out the mouth of the mine. On July 28, the powder charges were armed.