Archaeologists at the National Institute for Preventive Archaeological Research (Inrap) have uncovered what they describe as an "unprecedented" monument in Marliens, near Dijon, France.

Excavating the site, the team found artifacts from the Neolithic age to the Iron Age, including flint arrowheads, an archer's brace, and a copper alloy dagger. 

The oldest occupation of the site is the most intriguing, as it includes the strange monument, described as an 8-meter (26-foot) long "horseshoe" enclosure attached to a circular enclosure 11 meters (36 feet) in diameter, and another open enclosure attached to that. According to the team, layers of gravel in the side enclosures indicate the presence of structures, and all three of the enclosures appear to be from the same time period.

"This type of monument seems unprecedented and currently no comparison has been possible," Inrap explained in a press release. "The dating still remains uncertain, however the only artifacts discovered in the ditches correspond to cut flints which would suggest a chrono-cultural attribution to the Neolithic period."

The team is conducting radiocarbon analysis, which assesses the age of any formerly living object by measuring how much carbon-14 has decayed in its body after death, to get a better picture of when the monument was built.

Elsewhere at the site, the team found several wells dating from the Bronze Age, the only evidence of settlement from that period. A necropolis dated from between 1500 BCE and 1300 BCE by the presence of copper alloy pins and an amber necklace was also discovered, though the acidity of the soil means that no complete burial has been found by the team.

While intriguing, we don't yet know exactly when the monument was from. Carbon dating and further study will hopefully provide more clues to this weird, potentially very ancient monument and, perhaps, its purpose.