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What happens if one brings more than 10,000 USD with them into the US?
The $10,000 mark is not a ceiling in importing cash, but rather a point where an additional declaration needs to be made (Customs Form 4790). At 1 million, I suspect you might be in for a bit of an interview and delay. Here's an explanation of what happens when the declaration isn't made: https://www.cbp.gov/newsroom/local-media-release/failure-declare-results-seizure-24000-arrest-two