Building the cairns
There can be no doubt that the architectural and construction skills of the people who raised Orkney's chambered cairns were considerable.
These were not ignorant people and their apparent understanding of astronomy and mathematics is rivalled only by their ability to cut and dress the stone required to construct their monuments.
Building a chambered cairn required considerable effort. Working only with stone tools, the Neolithic craftsmen erected structures that are estimated to have taken between 10,000 and 100,000 man-hours to complete.
The sheer manpower required for the structures confirms their significance to the Neolithic people of Orkney. Stone blocks and flagstones had to be quarried, often from sites some distance from the site of the cairn, and where the chambers were cut into hills, such as Cuween, the main chamber had to be carved from solid bedrock before work on the roof could begin.
The builders' work, even by today's standards is impressive. Their drystane walls are neat and level and the interiors today, after many thousands of years, are still stable and dry.
In many cases, the cairns were also aligned with specific solar, and perhaps astronomical, events.
Most face north-east, the direction of the midwinter sunrise, but there are variations. Maeshowe, for example, is aligned so that the dying light of the midwinter sun shines through the entrance passage and strikes the back wall of the chamber.
Other theories link the chambers' position to astronomical occurrences such as the movement of the planet Venus.