system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know where to search for precious metals? How do we know that 100 ft underneath my yard isn't the largest gold deposit in the world?
gpt
Certain rock types are more likely to have certain minerals than others, but for the most part they look around where they found some precious metals and hope there is more.  Lot's of guesswork, gambling, and sampling.  Maybe you find nothing, maybe you strike it rich.