PERTH, Australia—Greg Cooke knows where gold worth hundreds of thousands of dollars is hidden. But like many people in this mining city, his problem isn't finding the precious metal; it is being able to recover it.

That's because the gold is in the form of dust that has accumulated in the brickwork of the old Perth Mint since its founding in the 1890s at the height of one of the world's great gold rushes. Decades of refining resulted in tiny fragments of gold embedding themselves in the fabric of the mint's historic melting house. To paraphrase a well-known bit of mining lore, there's gold in them thar walls!

"The gold in the walls isn't visible. You can't see it," said Mr. Cooke, a gold pourer at the mint. "But the moment you point it out to people, you see their eyes darting around the room with excitement."

Security is unobtrusive in the melting house, which was decommissioned in 1990. On any given day, tourists seeking a fix of Australia's gold rush history wander around the room pretty much undisturbed though they are within spitting distance of the secret hoard.

"I'd love to have a scrape," said Sacha Hibbitt, a 19-year-old student from England who was visiting Australia with a friend. "I like gold, and it would be nice to sell to pay for our trip."