The 197ft (60m) long ribbon of stones, found in Rotherwas, near Hereford, is thought to date from the same period as Stonehenge, roughly 2000 BC.
County archaeologist Dr Keith Ray said as far as he is aware the stone feature is unique in Europe.
"We can only speculate it may have been used in some kind of ritual," he said.
'International significance'
The Rotherwas Ribbon, as it is being called, is made up of a series of deliberately fire-cracked stones and appears to have been deliberately sculptured to undulate through the whole of its length that has so far been uncovered.
"This is an exciting find, not just for Herefordshire and the UK, but apparently, so far, unique in Europe. It has international significance," Dr Ray said.
Archaeologists said although the practice of laying stones in small level pavements is known at sites in Pembrokeshire and elsewhere, the closest parallel to the Rotherwas Ribbon is the "Great Serpent Mound", in Ohio, USA, which is thought to have been built between 200 BC and 400 AD.
The Serpent Mound is a 1,330ft (405m) long effigy of a serpent.
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