A multimillion pound windfarm could be scrapped after a Stone Age monument was spotted on the site using Google Earth.
Work to install the 15 wind turbines had already began after experts said they were unable to find anything of historical interest on the mountaintop in Carmarthenshire, Wales.
But a weekend rambler stumbled upon a row of stones while trekking across the site on the mountain and realised they were of historical interest.
Archaeologists were called in and discovered the stones on Mynydd Y Betws were between 3,500 and 5,000 years old and could have been part of an ancient site of worship.
Using Google Earth to plot the line of stones, experts claim the 1,600 ft-long monuments could be "almost as important as Stonehenge", freelance archaeologist Helen Gerrard told the Daily Mail.
Cambrian Renewable Energy, which was building the turbines, was working with the Welsh heritage organisation Cadw to assess whether the stones were used as part of a Stone Age monument.
The British Archaeological Trust was demanding a full archaeological survey of the mountain.