LONDON -- For a monument that has been drawing crowds for thousands of years, Stonehenge still holds many secrets.The stone circle, whose giant pillars each took 1,000 people to move, was erected between 5,000 and 3,500 years ago on a windswept plain in southwest England. Its purpose is still debated: Was it a solar calculator, a cemetery, a shrine?A new exhibition at the British Museum in London unravels some of the mystery — Stonehenge was, at times, all those things.