Hitler's secret Nazi war machines of World War II

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Heinrich Himmler, head of Hitler's Gestapo and the Waffen-SS, stands next to Adolf Hitler.

Earlier this week, the world reflected on the 76th anniversary of the start of World War II - when Hitler's Nazi army invaded Poland.

Hitler's engineers secretly developed some of the most ambitious projects and rapidly produced sophisticated technology decades before its time.

In the 2015 fall issue of Weapons of WWII magazine, author KM Lee detailed a few of Hitler's advanced weaponry.

Here's a look at are some of the secret, lethal weapons the Nazi's created during World War II: