NORTH KOREA AND SOUTH KOREA
Length: 150 miles
Why it's so dangerous: Military border fortification gone wild with almost 2 million troops, plus North Korean nukes.
Background: Since 1948, the 38th parallel has marked the division between North Korea and South Korea. At the conclusion of the Korean War in 1953, each side agreed to move its troops back from the front line, creating a 2.5-mile-wide buffer zone known as the demilitarized zone (DMZ). Although the two Koreas agreed to a cease-fire, no peace agreement or treaty was signed, and therefore the two sides are technically still at war.
Today: While the DMZ has been largely peaceful in the almost six decades since the end of the Korean War, it remains one of the world's most heavily militarized borders, with nearly 2 million soldiers patrolling both sides of the border. The past 18 months have been marked by an escalation in violence. A North Korean submarine reportedly sunk a South Korean warship in March 2010, and both sides exchanged fire after North Korea fired artillery at the South Korean island of Yeonpyeong in November 2010. Tensions are so high that on June 17, South Korean soldiers fired rifles at a South Korean commercial aircraft flying near the border after mistaking it for a North Korean jet fighter.
The United States, which maintains 28,000 troops in South Korea, has backed its South Korean ally to the hilt. Following joint military drills in February, North Korea accused South Korea and the United States of plotting to topple the North's communist government. Pyongyang threatened to start a "full-scale" war, take "merciless counteraction," and turn Seoul into a "sea of flames" if provoked any further.
JUNG YEON-JE/AFP/Getty Images