Chronic contained rupture of aortic aneurysm is a rare but important subset of ruptured aneurysms, which has a presentation that is distinctly different from that of acute rupture. These aneurysms are usually small. Patients are hemodynamically stable, usually presenting with a long history of back or loin pain, with symptoms attributable to compressive or erosive effects. The resultant pseudoaneurysmal sac may cause extensive vertebral erosion, but rib erosion has rarely been described previously. Even more unusual is the presentation of the pseudoaneurysmal sac as a large back mass. We present an unusual case of contained rupture of a thoracoabdominal aortic aneurysm, which presented as a back mass, and demonstrate the radiographic and computed tomography findings. Radiographs of the chest show large posterior mediastinal mass eroding the rib. Computed tomography scans demonstrate fusiform dilatation of the thoracoabdominal aorta with positive aortic drape sign, which is highly suggestive of a contained leak. The importance of early identification of this condition lies in the imminent danger of exsanguination, if undetected.