During the renovation of St. Johannis Protestant Church, construction work on a new underfloor heating system in the summer of 2013 accidentally uncovered the remains of a floor dating back to the 9th century. Experts have long assumed that the predecessor to St. Johannis Church dates back to the time of Hatto I, Archbishop of Mainz in the 9th century. It is considered highly probable that St. John's Church served as the cathedral church of the Archdiocese of Mainz before the current cathedral was built. St. John's Church has long been referred to as the "Old Cathedral." The current findings have now confirmed this.

In addition, wall remains were discovered in several places, indicating the foundations of another predecessor building (before Hatto), probably from the early Carolingian period (7th or 8th century). Some of the walls are still preserved up to a height of ten meters, which is unusual according to state conservator Joachim Glatz.

Experts are certain that St. Johannis is one of the oldest churches in Germany. Whether it is the oldest church is not yet known. Remains of a building from Roman times were found under the Vor-Hatto building, which could be an early Christian community from Roman times or a Roman temple. However, it is unclear what the Vor-Hatto building looked like.

The nave, which incidentally has a similar layout to churches in Rome, currently resembles a huge archaeological excavation site. The sense of space was completely different in Carolingian times because the historic floor was almost three meters below the current floor level, meaning that the church had completely different dimensions. It was a bipolar church with an altar in the west and in the east. There was a small central nave and a transept with transverse arms in the west. The side aisles were separated later. The overall height has been largely preserved. The building was significantly longer in the east. The large windows and round windows date back to the Hatto period. A crypt is suspected to exist. In addition, the color scheme was completely different.

In the basement, not only were the excellently founded outer walls of the church from the time of Charlemagne and the remains of a previous Roman building found, but also a sarcophagus without a lid and a burial site, each containing a skeleton. The graves date from different periods, with the burial site probably being older. One of the skeletons could date from the 7th century. Experts believe that these must have been prominent secular or clerical figures.

The state archaeology department is currently working on site in coordination with the Protestant Church. It is not yet clear how long the excavations will continue. What is certain is that the church will be used as a church again in the medium term, but that the archaeological finds will also be presented to the public. A concept is currently being worked on. An application to become a National Heritage Site is being submitted. Church services are currently being held in the parish hall and in neighboring churches.