We can literally sweep platinum off our highways, as Cody'sLab demonstrates[0]. Platinum is a catalyst in auto mufflers to change NOx molecules back to N2 and CO2. And a tiny amount flakes off as a gas-car drives. Sweeping a 10-foot section of heavily-used freeway shoulder netted them $0.55 of platinum. At this rate, there should be $1000 of platinum per mile of freeway shoulder.

[0] https://youtu.be/v5GPWJPLcHg