more (at source): Of all the tools in a military-grade escape kit, the most critical item was the map. But paper maps proved too fragile and cumbersome, said Debbie Hall, a cataloguer in the map room at the Bodleian Library at the University of Oxford in Oxford, England.

For hundreds of years, even before World War II, silk was the material of choice for military maps, Hall said, because it wouldn't tear or dissolve in water as easily as paper and was light enough to stuff into a boot or cigarette packet. Unlike maps printed on paper, silk maps also wouldn't rustle and attract the attention of enemy guards, she said.

How clever.