With trench warfare around Petersburg, Virginia stalemated, Union forces planned to blow up a section of Confederate line and make a breakthrough infantry assault. Weeks of tunnelling by Pennsylvania troops placed around 8000 pounds of explosives twenty feet below enemy positions. Before dawn, the resulting blast blew a massive crater into which Union infantry charged. The attack became a disaster as Confederate reinforcements closed the line, firing down on the Union infantry, which lost more than 3500 men. (By John Osborne)