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“A lot of people assume we’ve excavated the entire site and everything we’re ever going to know about the monument is known,” said Susan Greaney, from English Heritage.

“But actually there’s quite a lot we still don’t know and there’s quite a lot that can be discovered just through non-excavation methods.

“It’s great that people who know the site really well and look at it every day were able to spot these parch marks and recognize them for what they were.

“We maintain the grass with watering when it’s very dry in the summer, but our hosepipe doesn’t reach to the other side of the stone circle. If we’d had a longer hosepipe we might not have been able to see them.

“It’s really significant, and it shows us just how much we still have to learn about Stonehenge.”

Historians have long wondered whether Stonehenge was an intentionally-incomplete circle, but countless high resolution geophysical surveys and excavations have failed to give the answer.

Staff only water the site during the driest weeks of summer, but the hosepipe doesn’t reach to the south-west quadrant where there is a gap in the circle.

Worker Tim Daw spotted the patches on the ground – now thought to be ‘stone holes’ – where the grass was more parched.

He said: “I was standing on the public path looking at the grass near the stones and thinking that we needed to find a longer hosepipe to get the parched patches to green up.

“A sudden light-bulb moment in my head, and I remembered that the marks were where archaeologists had looked without success for signs that there had been stone holes, and that parch marks can signify them.

“I called my colleague over and he saw them and realized their possible significance as well. Not being archaeologists we called in the professionals to evaluate them.

“I am still amazed and very pleased that simply really looking at something, that tens of thousands of people had unwittingly seen, can reveal secrets that sophisticated machinery can’t.”