Don't believe the name: The Demilitarized Zone between the two rival Koreas might be the most heavily armed place on earth. Two million mines, barbed wire fences, tank traps and tens of thousands of troops from both countries patrol a divided swath of land 248 kilometers (154 miles) long and 4 kilometers (2.5 miles) wide.

So how, on the eve of Russian President Vladimir Putin's summit Wednesday with North Korean leader Kim Jong Un, did as many as 30 North Korean soldiers wander over the line separating North from South, causing South Korea to fire warning shots before the North Koreans withdrew?