The catacombs of St Paul and St Agatha are early Christian burial complexes just outside Mdina, in Rabat. Their underground chambers, passages, and tombs give a vivid look at late Roman and early Christian Malta.
St Paul's is the larger and better-known site, while St Agatha's has painted decoration and a more intimate feel. Together they are among the most worthwhile sights around Mdina if you have any interest in archaeology.