The Museum HAS conducted excavations next to the church of St. Michael in Proložac Donji- Postranje from 1986 to 1997 in cooperation with the Local History Museum of Imotski. The majority of this site was destructed in the course of the construction of modern tombs. The remaining 500 square metres of unspoiled surface revealed remains of ancient structures, whose purpose is still not determined, but also ruins of an early Christian complex and an early mediaeval, Pre-Romanesque church. 160 ancient- late ancient, mediaeval and modern tombs were explored. The discovered collection of moveable monuments that date to the period from the 1st century to the 20th century illustrate the continuity of burials at the location of the former local graveyard and of life in the nearby settlement from the beginning of the Common Era until today.

The monuments are ancient stelae, fragments of early Christian, Pre-Romanesque and modern church furnishings and other architectural elements, standing tombstones, a rich collection of metal jewellery and tools, Roman, Byzantine, Ottoman, Venetian and Austro-Hungarian coins as well as pieces of glass and ceramic pottery. The most significant monument from this site is the stone-carved, two-sided Mithraic icon with an entirely preserved blessing inscription, the illustration of the Tauroctony scene on the obverse and the Mithraic Feast and Hunt on the reverse (photo 1 and 2).