The first hydrogen bomb detonates.

The historic test took place on Elugelab Island, part of the Enewetak Atoll in the Marshall Islands (Pacific Proving Grounds).

Key Facts About the Ivy Mike Test

• The Design: It was the first full-scale test of a multi-stage thermonuclear device utilizing the Teller–Ulam design. Unlike earlier atomic bombs that relied solely on nuclear fission (splitting atoms), the hydrogen bomb used a primary fission bomb to compress and ignite a secondary stage of liquid deuterium fusion fuel (fusing atoms).

• The "Sausage": The device was not a deliverable weapon but a massive proof-of-concept installation nicknamed "Sausage". The device and its required cryogenic equipment weighed 82 short tons (74 metric tons) and was housed in a corrugated aluminum building resembling a small factory. 

• The Yield: The explosion produced an immense energy yield of 10.4 megatons of TNT. This made it nearly 700 times more powerful than the "Little Boy" bomb dropped on Hiroshima, making it the greatest man-made explosion on Earth up to that point.

• Complete Annihilation: The explosion created a fireball roughly 3 miles (4.8 km) in diameter. Elugelab Island completely disappeared, blown apart by the blast and replaced by a deep underwater crater over a mile wide.

• Scientific Discovery: Analysis of the radioactive fallout from the blast led to the discovery of two brand-new chemical elements: einsteinium (atomic number 99) and fermium (atomic number 100).

The success of Ivy Mike fundamentally escalated the Cold War nuclear arms race, proving the feasibility of thermonuclear weapons and prompting the Soviet Union to accelerate their own hydrogen bomb program.