The border area between North and South Korea may be the tensest place on earth

US Secretary of State Rex Tillerson said on Friday it may be necessary to take preemptive military action against North Korea if the threat from its weapons program were to reach a level "that we believe requires action."

Tillerson's comments came after he visited the demilitarized zone separating North and South Korea - a heavily fortified strip of land that may be the tensest place on the planet.

Although the Korean War is technically over for the U.S., the North and South are still very much at war - maintaining guard towers and thousands of troops facing each other, waiting for the next invasion.

The buffer zone created by the 1953 Armistice between North and South is called the demilitarized zone (DMZ), although there's a huge military presence. This border is filled with fencing, mines, and troops on both sides with itchy trigger fingers.

This relic of the Cold War has seen plenty of very hot engagements: Over 300 American and South Korean, and almost 400 North Korean soldiers were killed in firefights in 1969, and there are numerous instances of infiltrators from the North being scared back only by the sound of warning shots.

We've picked out the highlights from National Geographic's Inside North Korea and other sources.