The Bookan chambered cairn, to the north-west of the Ring of Brodgar, lies in what is arguably one of the richest archaeological landscapes in Orkney.
Today the cairn survives as a dilapidated oval mound, approximately 16 metres in diameter.
Inside, some of the internal chamber divisions remain visible.
Originally excavated in 1861, the cairn is close to the Ring of Bookan – the ditch and bank "henge" monument at the northern end of the Ness of Brodgar.
The 19th century investigation revealed a rectangular central chamber surrounded by what appeared to be five smaller chambers.
Human remains were found in three of the side chambers, along with pottery and a flint ‘lance-head’.
After the 1861 excavation, it was assumed that because of the structure's unfamiliar design, it had to be a very early example of a chambered tomb. It was, therefore, given a classification of its own and more or less forgotten about.
But the long-held assumption that the Bookan cairn dates from the early Neolithic appears to be contradicted by the pottery found by the antiquarian investigators.
The description they left has since been interpreted as being more like the later Grooved Ware rather than the Unstan Ware found in the early Neolithic period.
The fact that the site has similarities in both layout and architecture to Maeshowe and house two in Barnhouse has also muddied the waters considerably.
A two-week investigation in 2002 revealed much about the structure - in particular that the antiquarians’ early investigations had merely covered the earliest phase of its history.
Speaking at the time, archaeologist Nick Card explained: "After the original tomb, which measured approximately seven metres in diameter, had fallen into disrepair, it was incorporated into a larger cairn around 16 metres in diameter and bounded by three concentric stone revetments.”
The 2002 excavations also highlighted a number of distinct differences between the Bookan and Orkney’s other chambered cairns.
Nick Card said: "Various aspects of the tomb's layout, like the arrangement of the side compartments around a central chamber and the removable side-chamber 'doors', seem more akin to Orkney's Maeshowe-type of tombs rather than the (earlier) stalled Orkney-Cromarty tombs, like Unstan."
But, he added, despite its similarities to Maeshowe, Bookan's size and architectural aspects remain noticeably different to other chambered cairns found so far in Orkney.