In 1874 cracks were noted in the apse of the Lateran Basilica which threatened the structureߣs stability. Their discovery led to the decision to investigate the condition of the walls and foundations. The works, begun in 1876, focused on the sector to the West of the apse and the Leonine Portico. They brought to light not only the foundations of the Basilica and portico, but also the remains of a preexisting edifice, the so-called ߢTrapezoidal Building/Insulaߣ. As the works progressed, a series of disagreements began to emerge between members of the commission regarding the fate of the apse and its mosaic decoration, executed by Jacopo Torriti in 1291. As director of the works in 1876, Busiri Vici endeavored to find solutions that would reconcile the different parties involved and culminated in the brilliantly envisaged relocation of the apse. These works included a plan for the subterranean portion of the Basilica that considered leaving in plain view the recently discovered ancient structures. This chapter discusses the highly detailed documentation produced by Busiri Vici and explains its importance for the archaeology of the Lateran.