Engineers have reopened a tunnel dug in 1968 that goes deep inside the ancient monument of Silbury Hill in Wiltshire. The tunnel was the last of many made over the centuries by archaeologists and opportunists exploring the site. Parts of Silbury Hill are thought to be collapsing because of all the tunnels, so engineers are planning to stabilise the 5,000-year-old structure, believed to be the world’s largest man-made prehistoric mound. It will take around four months and hundreds of tonnes of chalk. Heavy rains in May 2000 caused substantial damage to the hill, with the collapse of an 18th century shaft. [...]