The underground Cathedral Excavation Museum documents construction dating back to Roman times.
Relicts of a Roman villa and the foundations of the Romanesque cathedral are especially interesting. The impressing mosaics, walls, canals and small discoveries in the roman villa were dug out by the archeologists and can now be seen in the Cathedral Excavation Museum which is directly in front of the dome, right under the Residenzplatz.
The museum is part of the Salzburg Museum and only open in July and August.