Introduction: Current state

In mid-2013, significant archaeological discoveries were made at the church while preparing to install underfloor heating. This construction project led to extensive archaeological excavations. The plaster was largely stripped from the entire building, revealing in some areas tuff masonry from the 6th or 7th century still intact at a height of about nine meters. Since mid-2017, the church's interior has been completely emptied. This process uncovered that the current walls, up to a height of about 16 meters, belong to a Carolingian basilica.

The excavations extended several meters below the most recent floor level, revealing that the ground level has risen by 2.60 meters since the 10th century, mainly due to accumulated debris from solid masonry. The current floor level is likely the seventh distinct level over time. As of 2023, these archaeological investigations are still ongoing. Ongoing research has recently provoked several digital reconstruction projects around the building.

A brief history of St. Johannis Church in Mainz in German. Produced in 2016.

Despite work that has been ongoing for many years, the building is still operational and open to the public. It is one of the few places where visitors can see such advanced archaeological work in person and request a guided tour to understand the complex and multi-layered history the building tells. 

You can read more about the history of the building in an online article written by Dr. Gudio Faccani, who is leading the archaeological work on the building.
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BIBLIOGRAPHY
1. Alter Dom St. Johannis. (2016). Alter Dom St. Johannis. In: YouTube [Video].
2. Faccani, G. (2022). Mainz, St. Johannis : Einhundertundzehn Jahre Bauforschung in einer Kirche. In: C. Sapin, S. Bully, M. Bizri, & F. Henrion (eds.), Archéologie du bâti. Aujourd’hui et demain (1–). ARTEHIS Éditions. https://doi.org/10.4000/books.artehis.29594.
3. Mimietz, A. (2023). St. Johannis - Mainz. In: Sketchfab. [3D Model].