Callanish Stones, Milky Way Night Sky, Isle of Lewis, Outer Hebrides, Scotland

Seeing outer space from this stone circle on the outskirts of Scotland, it's easy to imagine prehistoric people gazing at the same shooting star-filled sky in ages long past—and we're not even sure how long. It's estimated that the standing stones of the Ring of Brodgar were placed between 4,000 and 4,500 years ago, but precisely dating the site has proved difficult even with modern methods. Digs on the Ness of Brodgar—the wider UNESCO site that includes the ring—have revealed dozens of structures as old as 5,500 years, so after decades of study there are still more questions than answers about its age. Not to mention its purpose—though like other stone circles, especially its southerly cousin Stonehenge, it's hypothesized to have been a tool for ancient astronomers.