Underground mining is used when the copper ore is not near the earth's surface. Miners dig a shaft into the ore deposit vertically or a horizontal tunnel (or adit) in order to do this. Mining then follows the vein of ore and the underground mine develops.
These jewelry items are not actually filled with gold. They are made of a base metal (usually brass or copper) covered by sheets of gold in a mechanical bonding process. Effectively a thick coat of gold: the gold content is 5% or 1/20 of the total weight. Use goldfilled items for your topoftheline jewelry.