An instrument that automatically records temperature at various depths in the ocean as it descends.
From Greek 'bathys' (deep), 'therme' (heat), and '-graph' (instrument that records). Developed in the 1940s during World War II to help submarines understand water conditions.
The bathythermograph was invented partly for military reasons—submarines discovered that different water temperature layers bent sound waves differently, creating invisible 'highways' where they could hide from sonar detection, completely changing underwater warfare tactics.
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