A retired soldier told me he used to know people who smuggled jewelries, hard drugs, guns and even people from middle eastern countries to Europe, specifically, Germany! Via planes! In fact, he said some goods were too valuable for them to "risk it" by land transports.
He also added that the aircraft were operated by experienced World War II pilots who specialized in flying at high speed in very low altitudes to make deliveries for infantry units on distress, making them unnoticed on radars and unstoppable for air defense systems.
Are similar methods still possible to this date? if not, what has changed?
NOTE: I have researched myself and so far couldn't find evidence that any sort of actual "improvements" were done in these 70+ years in this matter, it seems the security tightens only when it's demanded and new technologies are not consecutively being used to monitor the skies.
no duplicate of Can someone actually "fly under the radar"? (read comments)