öbekli Tepe, in what is now southeastern Turkey, was built from about 9500 BCE and used until at least 8000 BCE. Big T-shaped stone pillars, some carved with animals. They are among the oldest known megaliths anywhere, older than farming in that region, older than writing by thousands of years. UNESCO listed it in 2018.

Nobody wrote down what it was for. People argue, carefully. We like that it's allowed to stay a question.

(the pillar drawing is a homage, not a picture of a real one)