Trajan’s Column, which is one of the great monuments of Roman antiquity, has been obsessively documented over the centuries by historians and archaeologists, and admired by tourists and Romans alike. Trajan’s Hollow exposes, and renders material, qualities of the Column neglected amidst all this attention. Sparked by Piranesi’s renowned engravings of the Column and the perennial tension between classical geometry and picturesque ruin, Stein’s research traces—and ultimately reconstructs as architecture—the missing monument, one overlooked by these competing ideals. This book reconstitutes the chiseled, eroding interior of the Column and reclaims its progeny—casts and copies of the original produced over two thousand years. Charting the Column’s extraction and its reproduction networks from the marble quarries of Carrara to the plaster diaspora of Paris, Trajan’s Hollow proposes a new ethos of scanning and replication, saturating digital technologies with an expansive material awareness to amplify the projective capacity of historical inquiry.