Archaeologists excavating the ruins of a medieval castle in western Hungary have uncovered an unusual stone-lined shaft showing evidence of intense heat, but its original purpose remains a mystery.

The structure was discovered during archaeological work at Essegvár Castle in Bánd, in Hungary’s Bakony region. Researchers from the Laczkó Dezső Museum have been investigating the site with university students and volunteers, uncovering evidence spanning several periods in the castle’s history.

The newly discovered shaft has emerged as one of the most puzzling finds from the latest excavations.

Although relatively small, the shaft was carefully constructed, with large stones positioned on edge around its sides.

Archaeologists found clear evidence that the structure had once been exposed to substantial heat. Traces of burning or heating can be observed on its bottom and sides as well as within material filling the shaft.

What generated that heat — and why the structure was built — has yet to be established.

Researchers have considered whether the shaft could be connected with some form of industrial activity. Metalworking, including the processing of iron or bronze, is among the possibilities, but the excavation has produced no slag or melted material nearby that would normally support such an interpretation.

Its construction also differs from the forms normally associated with medieval brick, lime or pottery kilns.

For the moment, archaeologists are treating its function as an open question and are continuing to gather evidence. One important point has already been established: the shaft was in use before the castle walls were constructed.