The Undercroft beneath York Minster was revealed during a major structural restoration in the 1960s, when engineers discovered that the central tower was in danger of collapse. Excavating to underpin the foundations, they found layer upon layer of earlier occupation: Norman foundations, Anglo-Saxon stonework, Viking burials, and beneath all of it, the stone remains of the principia — the headquarters building of the Roman legionary fortress of Eboracum. The central pillar of the basilica's great hall is still standing, 1,700 years old, in a basement beneath one of England's greatest Gothic cathedrals.
The Undercroft is now a separate ticketed exhibition within the Minster complex, and it is one of the more remarkable things you can see in York. The interpretation is good and the physical remains compelling: standing in the Roman room while the organ plays above is an unusual experience. The Undercroft ticket also includes the Minster Treasury, which holds medieval silver, the Horn of Ulf (an 11th-century ivory horn), and other ecclesiastical treasures.