During which American Civil War campaign did Union troops dig a tunnel beneath Confederate troops to detonate explosives underneath them?

The story behind the answer

During the Siege of Petersburg in 1864, Union soldiers—many former coal miners from the 48th Pennsylvania—dug a 511-foot tunnel under Confederate lines, packed it with about 8,000 pounds of black powder, and detonated it on July 30. The blast created the “Crater,” but the follow-up attack was bungled: Union troops surged into the pit, became trapped, and suffered heavy losses, squandering a chance to break the siege.

Called the Battle of the Crater, this moment captures Petersburg’s trench warfare and foreshadows World War I tactics. Memory tip: Petersburg = trenches = tunnel mine = the Crater. (Gettysburg and Antietam were open-field battles, not sieges; while Vicksburg was a siege, the most famous mining explosion in Civil War lore occurred at Petersburg.)