A massive machine — longer than a football field — is munching away beneath Washington like a giant earthworm. Before it’s done, it will devour about 2 million cubic yards of soil that has been sitting under the city since the days of the dinosaurs. It is the most amazing and expensive construction project that no one ever will see. It will come within a center fielder’s throw of Nationals Park, within a corner kick of RFK Stadium, nibble at the deepest roots from the National Arboretum, pass below the Love Nightclub and the United House of Prayer for All,...