Just by the salt water Loch of Stenness is the Unstan cairn, another Neolithic burial chamber. But this one you can just drive up to and explore on your own. There’s a gate over the entrance but it isn’t locked.
The chamber has a modern-ish roof and a skylight in it so it’s surprisingly bright when you get in there.
As you can see, this Neolithic burial chamber is partitioned off in places. These big slabs of stone are all over the island and have been used for buildings all over the place, very handy, they’re even used for fencing off fields sometimes. A lot of crouched skeletons were found in this burial cairn, along with a large number of pieces of pottery.
Quite a lot of the stones have been carved/grafittied over the years, some of it quite modern probably but the photo below might be of original carvings.
It’s well worth stopping off at Unstan Cairn if you’re visiting Orkney.