5. During the World War I, the German, French, and British troops used hollow trees to spy on their enemies.
© pikabu / @Interslav © rarehistoricalphotos
First, the craftsmen made special sketches and took measurements of a tree that was growing near the enemy’s territory. Then, they made an exact copy of that tree. During the night, a team of engineers carried out the uprooting operation and replaced the tree with a fake one. A soldier was hiding right inside of it and could carefully observe the enemy’s movements.