As global leaders turn their attention to the 75th anniversary of the United Nations and the 2020 General Assembly, GZERO Media offers a look back at one of the greatest diplomatic mysteries of the 20th century. The UN's second Secretary-General Dag Hammarskjöld's mysterious death in 1961, while on a mission to the Congo, is the subject of a new book by investigative journalist and former New York Times correspondent Ravi Somaiya. It has the twists and turns of a Tom Clancy novel.