Only fragments remain today and most are held in the Capitoline museum. They cover just 10 percent the original map surface that once stood on a wall in the Templum Pacis (Temple of Peace). The wall still survives today in a building near the 6th-century Church of Santi Cosma e Damiano. Rows of holes where the map was attached using bronze clamps can still be seen. Carved on 150 marble slabs, the 60-foot by 43-foot map detailed every building, street and staircase in Rome until it was partially ripped from the wall, probably to make lime for cement. What was...