Applying the principle of expressing structural forces to the unusual task of excavating, lining and holding open a huge ‘box’ at Westminster Underground Station creates a dramatic subterranean experience. Westminster Underground Station presented the challenge of humanising a piece of pure engineering. Our largest such challenge so far, a large hole, 32 metres deep, alongside Big Ben, the River Thames and underneath the new parliamentary offices. Co-ordinating the structures of station and offices allowed the ground floor atrium roof to become part of the transfer structure, freeing up space just below the ground for two levels, a spacious new ticket hall and the existing District and Jubilee Line tracks. The ‘box’ leads down to the new Jubilee Line platforms whose pedestrian tunnels are clad in the iron ribs with enamel infill panels used along the line. Subterranean structures behave differently to those above ground, but we remain true to our belief that expressing construction is the starting point for design. The box’s concrete diaphragm walls were poured against the side of the hole as it was excavated; but not being strong, they are supported by a grid of smoother concrete, adding an archaeological sense to the geological texture of the walls. Spanning between the walls are massive steel beams to resist lateral forces, and woven within them are the escalators and stairs, whose delicately detailed casings betray that this is a public space for the 21st century, rather than a Piranesian dungeon.