Staying in Scotland for a moment, the Clava Cairns is also a popular topic with mystery enthusiasts. It is a Bronze Age cemetery - one which was 'welcoming' residents four thousand years ago, and remained in use for well over one thousand years.
It would be historically significant even without a mystery because it's so well preserved, but it's the carvings at the site which people can't decipher. There are multiple indentations and cup marks on the stones, and their significance is a riddle.