A Borehole is a deep tunnel produced by terraformers. It can produce 6 minerals and 6 energy (7 if built on a river).
It does considerable eco-damage, causing the planet to fight back with fungus blooms and attacks by mind worms and other lifeforms.
If you have enough boreholes built you can raise the planet's temperature, and melt the icecaps in increasing amounts, flooding more coastal land.
The secret project Weather Paradigm allows you to build boreholes before you research the technology that would normally allow them, but without certain technology to remove the limits on energy and minerals, you can only get 2 minerals and 2 energy from them. Environmental Economics (SMAC) eliminates limits on energy. Ecological Engineering (SMAC) releases limits on minerals.