Beneath the Stones

From publisher blurb:

The Mare and Foal standing stones, just outside the town of Haltwhistle, are the only two stones remaining of an ancient stone circle. It is far from the best-known stone circle in Britain, but it marks an ancient place of geomantic power, being a place with a claim to be the exact geographical centre of Britain.

The stones are not impressive, and property developers have their eye on the site. Despite environmental protests, they now have the right to develop it. But before the developers can move in, a team of archaeologists has permission to excavate.

Now the town of Haltwhistle has been swallowed by dense mists. No word has got out. What could have happened?