In a development that will surprise absolutely no one who has ever tried to solve a prehistoric puzzle, the origin of Stonehenge's altar stone remains stubbornly elusive. A hardy band of geologists spent countless hours on weather-battered Scottish hills, only to conclude that the six-tonne slab probably didn't come from north-east Scotland after all - and that they may have been looking in the wrong spot entirely.

The saga began in 2024, when specialists from universities in London, Wales and Australia examined grains of sandstone from the altar and hypothesised it came from north-east Scotland, more than 400 miles from Stonehenge. One team member called the find "jaw-dropping," sparking speculation about how the stone was transported - by sledge or ship? - and about the long-distance relationships of neolithic people. More experts from Canada, Italy, Sweden and Scotland were drafted in, and the team conducted non-invasive examinations of the stone in situ and examined eight fragments held in museums.

Armed with this profile, the geologists headed to north-east Scotland to find an exact match with outcrops there. Nick Pearce, an emeritus professor of geochemistry at Aberystwyth University, described the hunt as a search for a needle in a haystack. "The problem is, it's a very big area, thousands of square kilometres. The only way to do it is to get out there and find the samples," he said. "One of my colleagues was there recently and he got caught in the storms, which isn't ideal field weather for geology. We've taken about 70 samples, brought them back to laboratories and tested them."

Testing suggested it is unlikely the altar stone came from north-east Scotland. Pearce insisted he wasn't disappointed: "We're getting closer. In a geological detective story such as this, ruling out potential source areas is just as important as identifying new ones. Every location eliminated narrows the search and brings us closer to discovering where the altar stone originated. I'm not disappointed. It's part of the process."

Duncan Pirrie, an emeritus professor at the University of South Wales, who led the study, concluded it was "very unlikely" the altar stone originated from north-east Scotland. He added that the stone definitely did not come from southern Britain. It may have come from somewhere else in Scotland, northern England or perhaps even Ireland. "This is the most comprehensive geological investigation of the altar stone ever undertaken," Pirrie said. "By combining a wide range of analytical techniques, we have built the clearest picture yet of the stone's composition and tested the leading theories about its origins. So far, none of the locations examined have the same set of characteristics as the altar stone, but the search is not over."

Mike Pitts, a renowned Stonehenge expert not involved in the study, said detecting the origin of the altar stone was an exciting prospect. "Doing so would greatly help fit the altar stone into the world of the people who moved it. We'd get to see the landscape through their eyes. To know exactly where the journey began." For now, though, the altar stone remains an enigma - and the geologists remain undeterred, if a little weather-beaten.