Years ago, KvonM wrote and published Waybills to El Dorado. It's out of print today, but copies can be found. It has twenty-five stories of plants that had not been found. Many years later, it is my guess that what could be recovered has been. But each story is valuable. There is one about a 19th century California politician who probably hid a fortune at his second home at Lake Tahoe. Well, above is the story of 20th century Paul Powell in Illinois. The differences are time and place. The essentials remain the same.
"Jesse Rascoe" (Ed Bartholomew) wrote several classic books that contain reliable information from 19th and early 20th century newspapers, and pioneer accounts, of lost mines and treasures throughout the west. He was a rare "treasure" writer who presented facts, not just what somebody else had penned before him. He also wrote Empty Money Pits; Or, Texas Treasure Finds (1974). It is just what the title states - and each recovery is an example of what remains to be located. Send me a PM if you're interested and can't locate a copy.
The best work on the subject is Harvey Bissell's Characters & Cache Planters (2021) - over 500 pages of solid leads. For years bootleg copies of his typescript were circulating through the professional treasure hunting community. Bootleg copies were sold for $100 and up. Finally, George Shiotelis did the heavy lifting to see Bissell's fine work in print. At some $50 it isn't cheap. It is an investment that should pay dividends to the careful reader.
Start documenting the finds you read about and the leads you come across. Write down the Who, Where, What, Why, and When. You'll begin to see patterns emerge. Human nature hasn't changed in millennia.
Good luck to all,
The Old Bookaroo