In the meantime, after General Grant had found it impossible to penetrate the lines established by Lee north of Richmond, he had crossed the James River south of Richmond and had set up camp on the outskirts of Petersburg. On June 15, 1864, Confederate General Beauregard held the forces under Grant back for three days until reenforcements under Lee arrived. Fighting continued between these two groups until April of the following year. During this siege, the Union forces decided to make a tunnel under the Confederate defenses which surrounded the city of Petersburg and to blow up the fortifications via a mine blast. Some Pennsylvania coal miners in one Union regiment were assigned the task of making the tunnel and laying the mine. On July 30, 1864 the mine was exploded, and the shape of the area after the explosion resembled a huge crater of a volcano. For this reason, this action is often referred to as the Battle of the Crater. Two hundred Confederate soldiers lost their lives due to the explosion. However, as the Union soldiers were ordered to charge up the sides of the recently carved crater, they found them too steep and while they were struggling to get out of the crater, about 4,000 Unionists lost their lives. The Union men could not see from behind the lines and continued to advance according to their orders into the crater until the crater was practically filled with struggling Union soldiers. The remaining Unionists were driven at the point of bayonets out of the crater back to their own lines. While the Petersburg siege was taking place, Lynchburg became the next objective of the Union forces. When General Jubal Early reached Charlottesville on his mission to block Union General Hunter, he learned that Hunter was heading for Lynchburg, the chief center of supplies of the Army of Northern Virginia and its center of communications. Early's forces in this battle consisted of two extremes in age: very young men cadets from the Virginia Military Institute and older men whose hair locks were so white that they were nicknamed "Silver grays." The fighting lasted from the afternoon of the 17th to the end of the next day when Hunter withdrew unexpectedly to the west. Early pursued him down the Shenandoah Valley and across the Potomac River to the outskirts of the Federal capital. Since Early, however, was not prepared for a fight on Washington, D. C., he returned to Winchester.