Which of the achievements of mankind in the twentieth century do you consider the most significant?

First of all, the most significant of all any achievements whatsoever is the beginning of human space exploration. This significantly changed the mood of people, brought science to a new, previously unknown level. The second of the significant achievements is the invention of computers, and as a result, the world wide Internet. This is not an achievement, but one might say, a technical revolution. Most interestingly, both of these achievements were made during the Cold War.