Top 5 scientific discoveries in the 20th C

[Gandahar]

The 20thC was a great Century for scientific discoveries, at the turn of the century people traveled by horse and cart and canals on Mars were the topic of the day. Atoms were still whole atoms and looked like a pie from the bakers shop.

Given 118 years of hindsight, what would be you top 5 scientific discoveries of the 20thC?