All mines need regular reinforcement against collapse, and Hallstatt, the world's oldest salt mine perched in the Austrian Alps, is no exception. But Hallstatt isn't like other mines. Exploited for 7,000 years, the mine has yielded not only a steady supply of salt but also archaeological discoveries attesting to the existence of a rich civilisation dating back to the early part of the first millennium BC. So far less than two percent of the prehistoric tunnel network is thought to have been explored, with the new round of reinforcement work, which began this month, protecting the dig's achievements, according to...