1986DA
This is a near-Earth object that seems to be metallic, and it's about 2 km in diameter.
I believe mining one of these objects probably ranks as one of the most profitable endeavors in history, combined with one of the highest up-front costs in history. In most other industries, there was a slow rise from small exploitation methods to larger, more complex methods.
Imagine if the very first oil well had to be a deep-water ocean rig.