The archaeological site of Šuplja Crkva (Hollow Church in Croatian) is the ruins of Burnum, a Roman empire encampment and city occupied between the first century BC and the sixth century AD.
The "Hollow Church" at Burnum; photo by Giuseppe.lepore4
Burnum was one of five camps in what was then Illyria, and located on the opposite side of the Adriatic sea from Italy, it was considered an important strategic location for Roman operations throughout eastern Europe, and as a backup plan for attacks on Italy.
Burnum was abandoned at the end of the Roman Empire. Archaeologists and related scholars have poked around there since the 1770s; but recent geophysical investigations by the Burnum Project have shown researchers even more about this ancient Roman municipality.