In the UK (though the law is different in Scotland) it's called 'treasure trove' (though that's not legally correct, I gather) and is assessed by coroners, whose normal responsibility is certifying deaths and reporting on unusual circumstances of deaths.
rnk> the govt owns any treasure you find.
The King, well "the Crown", I guess the £millions we give them each year aren't enough. One of the vestiges of feudalism, he has an army and we (the UK) don't so we apparently abandon reason, and democracy, ... I digress.
See the recently updated Treasure Act, https://www.legislation.gov.uk/ukpga/1996/24.