The Aydintepe underground settlement in northeastern Turkey is a series of subsurface galleries carved in Eocene-aged volcanic tuff. Rescue excavations conducted by the Local Archaeological Museum of Erzurum in 1996 and 1997 uncovered a large settlement with several underground rooms linked by passages. According to the excavators, it may have been built as a hideout and shelter by the first Christians who were banished by the Romans during the 3rd century A.D. Despite its historical and touristic value, parts of the underground settlement at Aydintepe have been destroyed to make room for the construction of new buildings in the modern town. As part of a conservation strategy for this important archaeological resource, a series of ground penetrating radar (GPR) and electrical resistivity measurements were conducted along eight profiles between 2003 and 2005. GPR soundings with a 100-MHz unshielded antenna yielded reflected hyperbolic signals with high amplitude from tunnel structures. These high-amplitude signals corresponded to areas of high electrical resistivity. These anomalies allow its to determine the probable locations of tunnels within the underground settlement and they provide important data that can be used to prevent future damage from modern construction. (C) 2008 Wiley Periodicals, Inc.