“Salting the Earth” is an idiom that references possibly apocryphal stories of ancient warfare where an invading army would literally put large amounts of salt or salt water in the enemy’s fields so that they wouldn’t be able to grow crops. This was done to make sure that the population couldn’t rebuild and become a threat in the future. Nowadays it is used to mean that someone is making really, really sure that something is destroyed and not coming back.
Edit: Part of the meaning is going out of your way in a big way to do it, because enough salt to actually have an effect, or digging a trench to get the ocean to fill the fields would have been astoundingly expensive.
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