Editing Battle of the Crater (section)

===Mine construction===
[[File:Petersburg crater sketch LOC.jpg|thumb|Contemporary sketch of Col. Pleasants supervising the placement of powder in the mine]]
[[File:NPS marker deicting details of the mine.jpg|thumb|National Park Service marker depicting details of the mine]]
Digging began in late June, but even Grant and Meade saw the operation as a "mere way to keep the men occupied" and doubted it of any actual tactical value. They quickly lost interest, and Pleasants soon found himself with few materials for his project, and his men even had to forage for wood to support the structure.

Work progressed steadily, however. Earth was removed by hand and packed into improvised sledges made from cracker boxes fitted with handles, and the floor, wall, and ceiling of the mine were shored up with timbers from an abandoned wood mill and even from tearing down an old bridge.

The shaft was elevated as it moved toward the Confederate lines to make sure moisture did not clog up the mine, and fresh air was drawn in by an ingenious air-exchange mechanism near the entrance. A canvas partition isolated the miners' air supply from outside air and allowed miners to enter and exit the work area easily. The miners had constructed a vertical exhaust shaft located well behind Union lines. At the vertical shaft's base, a fire was kept continuously burning. A wooden duct ran the entire length of the tunnel and protruded into the outside air. The fire heated stale air inside of the tunnel, drawing it up the exhaust shaft and out of the mine by the [[stack effect|chimney effect]]. The resulting vacuum then sucked fresh air in from the mine entrance via the wooden duct, which carried it down the length of the tunnel to the place in which the miners were working.<ref>Corrigan, pp. 36–37.</ref> That avoided the need for additional ventilation shafts, which could have been observed by the enemy, and it also easily disguised the diggers' progress.

On July 17, the main shaft reached under the Confederate position. Rumors of a mine construction soon reached the Confederates, but Lee refused to believe or act upon them for two weeks before he commenced countermining attempts, which were sluggish and uncoordinated, and were unable to discover the mine. However, General [[John Pegram (general)|John Pegram]], whose batteries would be above the explosion, took the threat seriously enough to build a new line of trenches and artillery points behind his position as a precaution.<ref name=blake53>{{harvnb|Blake|1935|pp=53–54}}</ref> Shafts were also sunk by the Confederates in an effort to intercept the passage.<ref>{{harvnb|Chernow|2017|p=427}}</ref> Pleasants became aware of the Confederate's counter-movements and was able to frustrate their effort by changing the direction of the main and lateral galleries while increasing their depth below the surface.<ref>{{harvnb|Blake|1935|p=54}}</ref>

The mine was in a "T"-shape. The approach shaft was {{convert|511|ft|m}} long, starting in a sunken area downhill and more than {{convert|50|ft|m}} below the Confederate battery, making detection difficult. The tunnel entrance was narrow, about {{convert|3|ft|m|0}} wide and {{convert|4.5|ft|m}} high. At its end, a perpendicular gallery of {{convert|75|ft|m}} 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 {{convert|8,000|lb|kg}}. The explosives were approximately {{convert|20|ft|m|sigfig=1}} under the Confederate works, and the T-gap was packed shut with {{convert|11|ft|m|0}} of earth in the side galleries. A further {{convert|32|ft|m|0}} of packed earth was placed in the main gallery to prevent the explosion blasting out the mouth of the mine. On July 28, the powder charges were armed.<ref name=chernow426>{{harvnb|Chernow|2017|pp=426–29}}</ref><ref name=blake53/>