Unstan is a burial chamber of a variety known as "stalled cairns", so named because of the large flagstones that divide the interior into separate stalls. It is believed to date to around 3400-2800 BC.
The interior is approached via an 8 metre entrance passage, which is low and narrow. Inside, the main chamber is around 8.5 metres long, and divided into five sections - three central stalls with two shelved compartments at either end. The whole thing is covered by a concrete dome that was added to protect the site in the 1930s. Externally, the tomb is covered by a circular grassy mound.
This is an interesting site, featuring wide, panoramic views out over the Loch of Stenness, taking in the whole sweep of the Orkney Neolithic landscape. The interior of the tomb is well-preserved, but while the concrete cap and skylight provide space to stand and take it all in, they do somewhat detract from the womb-like experience that's usually typical of such places.