The possibility that Newgrange could have a second passage tomb, which may also be aligned with a solstice event, is being explored by a team of Irish and Slovakian archaeologists who are using ground-breaking technology to probe the world-famous tumulus.
Already part of the Bru na Boinne World Heritage Site, Newgrange is one of the most popular tourist attractions in Ireland and if a second chamber is uncovered, it will add to its already global iconic status.
Newgrange is synonamous with sunrise on the winter solstice but the possibility that it has another as yet unknown chamber is not being ruled out. Indeed, the neighbouring mounds at Knowth and Dowth each have two passages.
"The absolute best case scenario would be to demonstrate there is an undiscovered passage and chamber within Newgrange because, despite how it may look, the mound has not been fully excavated," explained Dr Conor Brady, archaeologist with Dundalk Institute of Technology.
The north-west side of the mound has never been excavated so "it is technically possible there is something there on that side of the mound", he said.
After a week battling high winds and stormy weather, Dr Brady said: "The windy weather conditions prevented comprehensive coverage of the entire mound because the instruments are so sensitive. The early indications are that we did not identify another chamber of the same size as the existing one.
"There may still be a second chamber in the Newgrange mound, possibly smaller. We will know more later when the data collected are fully analysed. What we are absolutely sure about is the technique works and could be used to search for chamber in other mounds."
Fuelling the speculation of another chamber are local stories of when the caretaker of the monument opened up the tomb some 40 or 50 years ago and heard a big crash. She went into the chamber expecting to find it collapsed but it hadn't, "and whether there was something else in the monument that had collapsed is the question," Dr Brady added.
He is working with Prof Roman Pasteka from Comenius University in Bratislava, Slovakia, and Kevin Barton, an archaeology geophysicist as part of the project. "It is great for us to be involved in this type of archaeology. Newgrange is so well-known and we are very excited to be working at such a site. It is really an amazing place," said Prof Pasteka.
The funding for the week-long survey has been provided through Prof Pasteka and the Slovakian Scientific Academy.
His team has specialised in using microgravity surveying technology to detect underground cavities. Kevin Barton said the microgravity meter responds to variations in density in the ground beneath it.
If the surveys do confirm another passage, Dr Brady said: "It will change the way we think about Newgrange. It could be aligned with the winter solstice sunset; Newgrange will be a completely different entity after that".