Authorities in northwestern Spain are puzzling over how a concrete picnic table came to sit atop a prehistoric cemetery.

As first reported in La Region, a local newspaper in the region of Galicia, a pile of stones that sat atop a neolithic burial mound near the town of Cristovo de Cea appeared to have been removed earlier this year.

A local environmental group lodged a protest with the Galician public prosecutor's office in June, which has spurred an investigation. "The rolled concrete and modern picnic bench have caused irreparable damage, replacing what was a prehistoric cemetery of the first inhabitants of Cea," the group reported.

Galicia, perched on the northwestern corner of Spain, contains a multitude of neolithic and megalithic sites, including burial mounds and stone dolmens.

But José Luis Valladares, the town's mayor, expressed bafflement, complaining to the Huffington Post's Spanish edition that local officials didn't know of the presence of the ancient site.

"No one told me," he said.