Maes Howe

According to wikipedia this chambered cairn dates from around 2800 BC. On the day of the winter solstice, the sun sets over the top of the Barnhouse Stone, its last rays going on to illuminate the darkness of Maes Howe's inner chamber. Dowth and Newgrange on Ireland also have this function, though at Newgrange it occurs at sunrise instead. You have to bend down a bit when you walk through the passage, but once you're in the chamber, it quite spacious. Vikings broke into the mound in the 12th century, and they carved a whole lot of graffiti on the walls. The custodian is pointing out one of the inscriptions, and a little bit further to the left is another one that is visible : "That which I say will be true, that wealth was brought away. Wealth was brought away three nights before they broke this mound."

The runic inscription to the left says :
Ingibjôrg, the fair widow. Many a wife has travelled stooping in here. A great show-off. Erlingr. 
The other two pictures are part of a longer inscription that was carved by a couple of different persons :
This mound was constructed before Loðbrók's. Her sons, they were daring; such were men as they were of themselves (=they were the sort of people you would really call men).
Jerusalem-travellers broke Orkhaugr. Hlíf, the earl's houskeeper, carved.
In the north-west is great wealth concealed.
It was long ago that great wealth was concealed here.
Fortunate is he who can find the great riches.
Hákon alone carried wealth out of this mound.

This photo was taken from the hill just south of Finstown. There you can see Maes Howe and a few other monuments.