The Antiquities Act of 1906
The Federal Government, as well as every state in the Union, has numerous laws governing the status and protection of historically significant lands and objects within their jurisdictions. Objects over 50 years old come under this definition, and so care must be taken when treasure hunting, or laws may be inadvertently violated. It is wise to familiarize yourself with these laws before beginning a treasure hunting adventure. Here is a good source for the 1906 Antiquities Act. I suggest doing a search for "Antiquities Act" for the particular state you will be working in to get the proper information.