After centuries of speculation about the initial design of Stonehenge, the giant, prehistoric monument in England recognized worldwide, scientists say they now know the weathered stone semi-circle used to be full.

And, it wasn't advanced mathematical calculations or carbon dating that led to the insight, but patches of brown grass, reports United Press International.

In 2013, the story goes, after the earth around the monument site had become particularly dry, several brown, scorched swaths of grass appeared in pretty much the same locations an observer could have logically deduced missing stones were once placed.

"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," the UPI story said Tim Daw, the site's groundskeeper, told BBC News. "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."

So, Daw called local archaeologists to have a better look.

Experts now believe that a full Stonehenge circle was likely constructed in several phases during the Neolithic period, with the entire building process lasting several hundreds of years between 3000 BCE to 2000 BCE.

This means that Daw's assumptions were correct -- though archaeologists contend they've had limited time to make a firmer assessment.

What scientists really need is another dry spell again, so that the remnants of the damaged grass reappears.

"Ideally the survey would have differentiated between marks caused by parching -- the majority -- and those caused by lusher growth," scientists wrote in a study that's been published in the journal Antiquity.

"It would have also have graded the marks into 'definitive', 'probable' and 'possible' categories," the study's authors continued. "This was not possible, and the result must therefore be treated with caution."

Said Susan Greaney, historian with the Stonehenge care-taking and cultural preservation group English Heritage, in the BBC News story: "If these stone holes actually held upright stones then we've got a complete circle ... It's really significant, and it shows us just how much we still have to learn about Stonehenge."