“It is the most high-energy, intense city I have ever lived in. I’ve lived in New York City and Tokyo, but Seoul beats them hands down,” said Ruchika Sahai, who moved here from Sydney, Australia, nearly two years ago. Office hours stretch well past 10 pm, but the drive of the nearly 10 million residents rarely wavers. Even beyond the modern city streets, the underground markets pulse with a chaotic and old world vibe.