The Miles Stair is a 12-foot-wide helix of white concrete that winds through five stories of Somerset House, a cultural center in London. Staircases typically use surrounding walls for support, but the Miles Stair relies on a core built from a latticework of lightweight stainless steel. Engineers managed to pull off this improbable structure because the steps are built from high-performance concrete, which is stronger, lighter and more stable than regular concrete.