Escape from Germany: the greatest POW break-out of the First World War. Neil Hanson. 2012.

July, 1918. The most heavily guarded POW camp in the world. 

Surrounded by steel palisades and barbed-wire fences, patrolled by ferocious dogs and armed guards with orders to shoot to kill. Holzminden was a brutal punishment camp. To escape would take boundless ingenuity and nerves of steel.

Many tried. Prisoners used sardine-tin openers to pick locks, forged documents, sent messages using milk as an invisible ink, and created fake uniforms and elaborate disguises. Every attempt failed, leading only to ever-tighter defences.

But on the night of 23 July 1918, twenty-nine undaunted Allied prisoners achieved the impossible. They had spent nine months using cutlery to move tonnes of earth, clay and stone, digging a tunnel over 150 feet long under the walls and barb-wire fences, to the farmland beyond.