Borehole mines are resource buildings in Modern Times (Tropico 4) that take the place of mines. They exploit all veins of resources that are underneath the main structure. Because the hole runs so deep, borehole mines can continue to function and produce resources (though at a much slower rate) even after the vein it was originally built over is completely depleted. However, if a disaster destroys the mine there will be no option to rebuild it.
The Borehole Mine requires half the workers of a regular mine but produce at least as much as a fully-staffed mine, making them very workforce-efficient.