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This vibrant monument of the imperial age has been housed in one of Rome's newest architectural landmarks: a gleaming, rectangular glass-and-travertine structure designed by American architect Richard Meier. Overlooking the Tiber on one side and the ruins of the marble-clad Mausoleo di Augusto (Mausoleum of Augustus), on the other, the result is a serene, luminous oasis right in Rome's center. Opened in 2006, after a decade of bitter controversy over the monument's relocation, the altar itself dates back to 13 BC; it was commissioned to celebrate the Pax Romana, the era of peace ushered in by Augustus's military victories. It is covered with spectacular and moving relief sculptures. Like all ancient Roman monuments of this type, you have to imagine them painted in vibrant colors, now long gone. The reliefs on the short sides show myths associated with Rome's founding and glory; the long sides display a procession of the imperial family. It's fun to try to play "who's who"—although half of his body is missing, Augustus is identifiable as the first full figure at the procession's head on the south-side frieze—but academics still argue over exact identifications.