How would treasure hunters like Indiana Jones square with the IRS?

Let's say Nathan Drake finds $15 million dollars in gold bullion in a Spanish fortress in the middle of God-knows-where. He somehow takes it home to America.

If he were to sell all of the coins for 15 million in cash, where/how would the government get their take? Would they get the same take if he just kept them in a pillowcase under his bed? Would he have to list his income as 15 million dollars on his taxes?