A dark sky sits peacefully above the valley of the great river that has been a European border since time immemorial. Once the limes of the Roman empire, beyond which lay monsters and barbarians, the barrier was formed by a block of rock slipping down between two faults in response to pull apart tectonic stresses. The force came from the ongoing collision between the African and Eurasian plates and the rise of the Alps and Pyrenees. It started opening in the late Eocene, some 35 million years ago, and the faults are still active. Basel was levelled by a huge quake on October 18th 1356CE. River valleys often fill with fog on cold clear nights, since the air cools faster than the water, which evaporates. On the evening the photo was taken a river of mist masked the Rhine below, brilliantly backlit by town lights and creating a scene of surreal splendour.