The western side of the site with the timber-beam slots continuing beyond the excavation. So far the researchers have found they extend at least 50 feet long.

Archaeologists have unearthed the foundation of what appears to have been a massive, ancient structure, possibly a bridge leading to an artificial island, in what is now southeast Wales. The strange ruin, its discoverers say, is unlike anything found before in the United Kingdom and possibly all of Europe.

"It's a real mystery," said Steve Clarke, chairman and founding member of the Monmouth Archaeological Society, who discovered the structural remains earlier this month in Monmouth, Wales — a town known for its rich archaeological features. "Whatever it is, there's nothing else like it. It may well be unique."

Clarke and his team discovered the remnants of three giant timber beams placed alongside one another on a floodplainat the edge of an ancient lake that has long since filled with silt. After being set into the ground, the pieces of timber decayed, leaving anaerobic (oxygen-free) clay, which formed after silt filled in the timbers' empty slots, Clarke told LiveScience. [Photos of the mysterious structure]

The team initially thought the timber structures were once sleeper beams, or shafts of timber placed in the ground to form the foundations of a house. However, the pieces appear to be too large for that purpose. While a typical sleeper beam would span about 1 foot (30 centimeters) across, these timber beams were over 3 feet wide and at least 50 feet long (or about 1 meter by 15 meters). The archaeologists are still digging and don't yet know how much longer the timbers are. Clarke says the structure's builders appear to have placed whole trees, cut in half lengthwise, into the ground.

"One other thing that is striking, that might be relevant, is that the timbers seem to be lined up with the middle of the lake," Clarke noted, suggesting that the structures may have been part of a causeway to a crannog, or artificial island, constructed in the middle of the lake. "Even so, if it is a path to a crannog, it's huge."

The archaeologists also aren't sure when it was built or even if it came before or after the lake formed, but they say the structure, at its oldest, could date to the Bronze Age around 4,000 years ago. Beneath the beams the researchers found a burnt mound of rock and charcoal fragments, alongside of which they discovered a hearth and trough — scientists believe people in the Bronze Age heated stones in a fire and threw them into a filled trough to boil water.

"The discovery of this unusual site on a housing development near Monmouth is very interesting," a spokesperson for CADW, the Welsh government’s historic environment service, told LiveScience. "We have been monitoring the situation closely. At this point the date and function of the structure represented by these three long trenches is not known, despite a great deal of speculation. Only further excavation can clarify exactly what they represent."

Clarke believes its more likely the structure was built a little later, possibly during the Iron Age, but he says determining a reliable age for the structure will be tricky. Dating the burnt mound, which predates the timber that was placed on top of it, will only give a maximum age for the structure. Dating the clay, on the other hand, will yield an age that is too young because the clay deposited after the timber rotted away.

The archaeologists have already sent off charcoal samples from the burnt mound for chemical analyses and expect results later this month.

"And we now have some charcoal from the bottom of the slots (not from the burnt-mound area)," Clarke said. "Hopefully that will give us a closer date."

The research has yet to be published in a peer-reviewed scientific journal, with work at the site currently in progress.

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