Specifically in relation to the battle of france and the bypass of the Maginot line, while I've heard the allies were waiting in the North.
Why was the Ardennes considered impassable, and what did the Germans do that changed that?
Since I'm Belgian and have been to the Ardennes quite often I think I can answer:
Compared to the regions to the south, the Ardennes are more mountainous and difficult to employ tanks. Other than that there was Fort Eben-Emael, which was a huge fort that dominated the area (it had cannons with a shooting range of 10 miles). Other than that, the Ardennes are rather swampy sometimes. So it would've been very difficult to pass through the Ardennes unharmed for armored vehicles.
How did the Germans circumvent this? With superior air power. They quickly took out half the Belgian airforce when the invaded Belgium, and they took Fort Eben-Emael by landing inside it with parachutes. Roughly 90 soldiers were all that was needed to take control of what was then regarded as the most modern fort in Western-Europe.