> When treasure is found, the finder does not own it, and it is illegal to try to sell it, according to government guidelines.
There could well be guidelines that say that, about the law ('it is illegal', as it says). HMRC has plenty of guidelines on what tax you owe and how to pay it etc. for example, but it also comes down to law at the end of it.
[0]: https://www.gov.uk/government/publications/treasure-act-1996...
And I will forevermore know to call people operating metal detectors "detectorists."
https://news.ycombinator.com/item?id=29405696
(I'd thought this might be a dupe, it's not.)
Best way to get started would be to watch The Detectorists on Amazon.
The Mojave nugget was found in California with a radio shack metal detector https://en.wikipedia.org/wiki/Mojave_Nugget
The hardest part is getting permissions from land owners, many of whom have no idea what you're trying to do and are therefore rightly suspicious. But I'm fairly outgoing and friendly in nature so that hasn't been too difficult.
As others have mentioned, if you're in the US, you're unlikely to find very old things like European and Asian hobbyists are able to, but finding 150 year old coins and war relics is satisfying enough for me.
Land rights, mineral rights, and rule of capture are suggested terms for exploration.
Much of the law has firmer foundations in tradition than any consideration of science, empiricism, justice, or merits.