As we look across an urban landscape, sometimes we may wonder: "What might have been here?" Monuments and dramatic buildings are often part of the picture, and in a federal city, there are frequently many structures that are proposed, but never built. This exhibition from the National Building Museum "reveals the Washington that could have been by presenting architectural and urban design projects that were proposed but, for widely varied reasons, never executed." The site has information about the in situ exhibit, along with a fine introductory essay by curator Martin Moeller titled "What Should a Capitol Look Like?" Next up is the interactive "Unbuilt Washington" map, which allows users the opportunity to learn about noteworthy unbuilt projects from the 1790s to the present. Visitors shouldn't miss the rendering of the proposed executive mansion from 1898 created by architect Paul Pelz or the proposal for a National Stadium from 1932.