With the discovery of gold in 1848, thousands of prospectors poured into California. When it came time to create a state, the new residents wanted to make sure California included all potential gold fields in the Sierra Nevada range, and so drew their own borders.
George Washington picked out the location and laid out a perfect diamond shape for the city, spanning parts of Maryland and Virginia. But a looming fight over slavery caused the Virginia side to leave the District and return to its home state, giving D.C. its strange shape.