The original plan was that the archaeologists would research the already-known ruins of St Olof, St Per and St Lars from the middle ages in the town north of Stockholm.
But the project has led to an unexpected discovery.
Hidden in the ground under the ruins of St Olof’s church, which itself dates back to the early 12th century, are traces of another stone church altogether from an even earlier period.
The find is so surprising because the eleventh century has always been associated with the construction of wooden churches in medieval Sweden.
“I couldn’t in my wildest fantasy imagine that we would find the foundations of a stone church,” said Sten Tesch, an archaeologist and head of Sigtuna Museum, to news agency TT.
Until now Herrestad church in Östergötland, which is thought to have been built in the 1100s, was the oldest dated stone church in Sweden.
Archaeologists began digging at St Olof’s in 2001 and a number of small discoveries have led them to this conclusion.
A solid stone floor, a layout exposed a metre under the ground surface and graves even deeper are among the pieces of evidence which make the researchers pretty certain that they have found an 11 metre long and 8.5 metre wide stone church.
But the precise year of construction is one of several questions which still remain to be answered.
“The shape is a little unclear. The church is rather squat and there is no chancel. It’s possible that this was the chancel – perhaps the construction of the church was stopped,” said Sten Tesch.
Unlike Herrestad, it will not be possible to use dendrochronology – measuring the age of wood using the tree rings – to date the church, since no wooden pieces have yet been found.
Instead, researchers hope to pin down the age of the surrounding graves using carbon-14 dating.