THE BLACK stain on the ice was instantly recognisable. The technician checking a tarpaulin stretched over a section of the Presena Glacier in the Italian Alps—an experimental attempt to slow the melting— quickly called in a rescue party. The block of ice was airlifted to the nearby city of Vicenza. Inside were two soldiers who had fallen at the Battle of Presena in May 1918 and were buried in a crevasse....