Is found money taxable?

Before anyone congratulates me, I didn’t find any money (I keep hoping some drug smuggler will drop a duffle bag full of Benjamins from an airplane right onto my deck, but so far, no luck). I’m just curious, if you did find a significant chunk of money, like say more than you could just deposit into your bank and not have the manager report you to the IRS, let’s say you actually even turned the money into the police and no one claimed it and it was yours fair and square, would you legally have to report it as income?

If so, is there a lower limit on this or technically should we all be reporting it every time we find a five floating around in a parking lot somewhere? And if so, is it regular income or is it taxed at a different rate? For example, many years ago I heard that people who try to salvage shipwrecks if they make a big find have to fork over something like 70% of it in taxes.

And let’s say as a supplemental question, if we do establish that you would be legally obligated to pay taxes on the money (which I suspect is the answer), and you found a bunch of money, like say a million dollars, would you start paying for a lot of things in cash, make smaller deposits in multiple bank accounts under a certain threshold and basically try to conceal that you found it from the IRS, or would you just report it and fork over half of it to the Federal and State government?

Note, this question presumes you’re in the US, but of course if you live elsewhere and know of your country’s laws, feel free to share.