France's great bulwark of frontier defense was the powerful Maginot Line, equipped with the most modern devices of the time.  It was conceived by Andre Maginot.  Maginot was an ex-service man was France's War Minister.  The line was permanently occupied by specialist troops, who could be transported rapidly from one sector to another in secrecy and in perfect safety by means of the underground trolley system seen in the above photo.  Unfortunately the Germans didn't play by the rules and simply went around the Maginot line instead of attacking it.

To The Maginot Line by Underground

This photo taken in early 1940 shows what was then called an "Autogyro".  Autogyros were used by the French navy for submarine spotting, since this type of aircraft was capable of hovering over a defined area and could land in a very restricted space, such as the deck of a warship.  This Autogyro is seen on the landing ground of an areodrome near Toulon.