The find was far from total luck.What did the trick was ground-penetrating radar, which is also used in archeology. The nugget itself was about ten feet down in a crevice; after the radar was used, a metal detector did the trick. The man also found to much small, but still large, nuggets in the crevice he exposed.
The Nevada County resident had his undeveloped property – not far from the old mining town of Washington – assayed by a professional for possible gold deposits.
"Just to see what gold would be down to the first 10 feet," he said.
There was some fine gold, and a hint that there might be more in the bedrock beneath the old mining tailings.
Knowing how Gold Rush lodes were found in similar bedrock, "it led us to think there may be some," he said....
Below is a picture of the lucky fellow with his find, gotten from the same article linked to above: