1. How did people 1st map the ocean floor?

echo sound or sound waves

Explanation:

Echo sounding is the key method scientists use to map the seafloor today. The technique, first used by German scientists in the early 20th century, uses sound waves bounced off the ocean bottom. ... The time taken for the sound to travel through the ocean and back is then used to calculate water depths.

they were destroyed coming out of the civil war but after he war they were booming. if you need an in-depth answer follow up with me  

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