Great link
A summary from the article for those discouraged by the length:
Treasure trove law in the United States may be summarized as follows:
(1) Treasure trove goes to the finder, unless the finder is trespassing. If the finder
is trespassing, it goes to the locus owner.
(2) Treasure trove includes gold coins, silver coins, gold bullion, silver bullion,
plus paper money; it must have “the thought of antiquity,” i.e. be several decades
old. Courts have not yet decided whether treasure trove includes coins of base metals.
(3) Many finds of money are now handled not through the common law of treasure
trove, but through statutory schemes, under which the money is deposited with
the police for between ninety days and a year, and if the owner does not claim it by
the end of that period, the money vests in the finder.
(4) Two states – Tennessee and Idaho – award treasure trove to the locus owner.
(5) Treasure trove is taxable at the ordinary income rate in the year that it is discovered.
(6) Employees get to keep what they find when acting in their course of employment,
unless their employer has a heightened legal obligation towards the customers,
in which case the property goes to the employer. Examples of employers with
heightened legal obligations are hospitals vis-à-vis their patients, common carriers
vis-à-vis their passengers, and hotels vis-à-vis their guests.
(7) Police officers, baggage inspectors, and members of the armed forces do not
get to keep what they find when on duty.
(8) Finds in banks go to the bank; finds on government land go to the government.
(9) Finds in Indian graves on Federal or Indian land go to the Indian tribe that is
most closely related to the decedent.