If you discover an island covered in guano — old poop — an 1856 Federal law that's still on the books obliges the US of A to defend your claim to it.
The Guano Act of 1856 dates to an era when old poo was of national interest as a vital source of fertilizer, and it's documented in Kevin Underhill's delightful compendium of weird laws, The Emergency Sasquatch Ordinance (review, excerpt).
In short, the act allowed any citizen to claim an island on behalf of the United States, as long as that island was uninhabited and covered in bird poop.
Did anyone take advantage of that? They sure did. More than 100 islands around the globe were claimed this way, including Midway Island and about 99 islands you've never heard of. (This was a bigger deal than you might think, legally, because the precedent led to the whole concept of "unincorporated territories" or "insular areas" over which the U.S. exercises jurisdiction, but in which the Constitution does not fully apply.) The U.S. has given up any claims on most of those, but it still claims a handful and in fact actively opposes at least one claim by another nation (Haiti).
Can you take advantage of the Guano Islands Act today? You sure can. See 48 U.S.C. § 1411-19. In fact, somebody tried to use it as recently as 1997, trying for some reason to claim Navassa Island (the one near Haiti). But in Warren v. United States, the court held that Navassa has "appertained to" the U.S. since 1857, and the U.S. has never given it up. The real question, why anybody in his right mind would want Navassa Island, a waterless hellhole from which all the dung has already been mined, was not addressed.
(Image: Smelly Penguins, Salim Fadhley, CC-BY-SA)