For the last 250 years, the American story has taken place against a backdrop of not only sea-to-shining-sea natural wonders , but also vibrant urban landscapes. As the United States approaches its Semiquincentennial, TIME asked 25 architects, urban planners, thinkers, and other experts to each nominate a building or monument that they believe says something special about the nation at this moment. Their answers, encompassing both Washington’s most famous sites and humble homegrown architectural forms, paint a portrait of America in three dimensions. The Houston Astrodome stadium in 1965. Bettmann Archive/Getty Images The Houston Astrodome (Houston) Selected by Maria Nicanor Historic preservation and adaptive reuse are the future of architecture in our country and worldwide. For America, saving the structures that defined the country’s prowess in innovation, experimentation, and ingenuity means looking to a relatively recent past.…