The H of H-Bomb, First Tested in the US in 1952
Puzzle 9 of Deep Sea Journey Hard Pack
Question is: The H of H-Bomb, First Tested in the US in 1952
and the answer is: HYDROGEN
Word Meaning:
HYDROGEN — (uncountable) The lightest chemical element (symbol H), with an atomic number of 1 and atomic weight of 1.008.
Definitions from Wiktionary.