The Union hoped to end the Civil War by taking Richmond, but the Confederacy kept holding them off. The two armies eventually dug trenches facing each other south of Richmond in the town of Petersburg. (Military historians say that often times particular tactics employed late in one war presage what tactics will dominate in later wars. The Siege of Petersburg foreshadowed the trench warfare of World War I.)
Attempting to break the deadlock, the Union used Pennsylvania coal miners to dig a tunnel under the Confederate lines which they then packed with explosives and detonated. The contours of the resulting crater can still be seen today.
But the North squandered their advantage and the entire effort was a “stupendous failure”.
The horror of the Battle of the Crater was (loosely) depicted in the Anthony Minghella film Cold Mountain: