In the so-called "Old Cathedral" in Mainz, which is today the evangelical Church of St John, archaeologists found the remains of another church built 1,200 years ago in the time of Charlemagne, Deacon Andreas Klodt said on Tuesday.
Only Trier on the Mosel River has an older church, with its cathedral dating back to Roman times, making the find the second oldest church in the country.
Professor Matthias Untermann from the Institute of Art History in Heidelberg said the remains of the Carolingian walls stretched from the basement to the roof.
“This is a big surprise,” he said.
The Rhineland-Palatinate state curator Joachim Glatz said: "This is the only surviving Carolingian cathedral in Germany."
Usually a bishop would build a cathedral in the Middle Ages at the exact location of the previous building, getting rid of the older church.
But in Mainz the 1,000-year-old "Old Cathedral" was incorporated into the Carolingian one.
Archaeologist Ronald Knöchlein said Mainz had a Christian community in Roman times.
And according to Knöchlein two human skeletons have been found during the excavations - remains of earlier burials in the church.
The building has been used since the 19th Century as a church by the evangelical community.
During World War II, it was largely destroyed by fire after a bomb attack. Demolition was considered but was rejected. The dig is continuing.
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