In the church of Sankt Johannis in Mainz, formerly the city’s ancient Medieval cathedral, now an evangelical church, there was a sophisticated floor-heating system even as early as the eighth century. The archaeologists found the network of pipes dating back to 1300 years ago as they were engaged in the excavations and archaeological search that have been going on since 2013. In the four years the Johanniskirche has been closed, the Protestant community celebrated its services in the nearby church of the Catholic seminary. The director of the excavations, Guido Faccani, a Swiss archaeologist specialising in medieval studies, spoke of the role the church of Sankt Johannis has played since its foundation, in the fifth century. At present, Sankt Johannis is the oldest Early Medieval church north of the Alps, with large-scale well-preserved original walls. In 2013, the archaeologists had dug down to three metres under the floor. The excavations should finish soon, except in some places within the inner and outer boundaries. 400 thousand finds have been unearthed, including a lot of gravestones, dice and other pieces from the Roman and Stone ages. Many parts of a Gothic tiled floor have reappeared. The excavations found that the body of the ancient church was twelve to sixteen metres tall, and the current elevation and extension were built in 1300. Sankt Johannis was the Cathedral of Mainz at least until 998, when the then bishop Willigis was commissioned by Emperor Otto II to build a new one.