Archaeologists have found around 100 huge standing stones buried near Stonehenge.
Experts think the discovery could be the 'biggest' prehistoric monument ever built in Britain.
The stones were uncovered using special equipment that scans below the Earth's surface.
Researchers said finding the stones was "fantastically lucky".
What is Stonehenge?
- It's a circle of massive stones, on Salisbury Plain in Wiltshire, in the south west of England.
- No-one knows exactly why it was built but there is a link to the sun's movements during the year and there are burial sites nearby.
- It's thought to have been built in prehistoric times, with the stones being put up about 2,500 BC.
- People go to the site at certain times of year, during the summer and winter solstices, when the sun lines up with gaps in the stones.
The latest discovery was made about 1.8 miles (3km) from Stonehenge, on the edge of the Durrington Walls "henge", or bank.
It's an area which had not yet been studied by researchers and it's thought the stones may have originally measured up to 4.5m (14ft) in height.
Lead researcher, Vince Gaffney said the stones were "lost to archaeology" but found thanks to modern technology.
The findings are being announced later on the first day of the British Science Festival being held at the University of Bradford.