Seoul

Braess's paradox describes how building new roads can slow down traffic. When every driver picks their optimal route, the collective result worsens. When Seoul's Namsan Tunnel No. 2 was closed in 1999, the city's average road speed actually increased.

In 1968, Seoul dynamited Bamseom island to develop Yeouido. But Han River sediment accumulated on its bedrock, and the island resurrected — growing even larger. Today Bamseom is an uninhabited ecological reserve designated as a Ramsar Wetland.

In 1976, ten bright lights appeared in formation over Seoul. The military fired anti-aircraft guns, but the lights drifted away unscathed. Stray rounds killed one civilian and injured 31. Officials blamed a US cargo plane, but many questions remain unanswered.