15: nonrenewable energy sources, their impacts, and energy. Fossil fuels -> #1 energy source, highly combustible substances formed underground over millions of years from buried remains of ancient organisms: solid = coal, liquid = oil, gas = natural gas. Electricity -> secondary form of energy that we can transfer over long distances and apply to many uses: 2/3 comes from fossil fuels, most industrialized countries use 100 times more electricity than least. Net energy -> energy returned subtracted from energy invested . Eroi (energy returned on investment) -> energy returned / energy invested : high ratio = we receive more energy from each unit of energy that we invest, fossil fuels have high eroi ratios, ratios can rise and fall. Harder to extract, technology and efficiency changes, etc. Anaerobic environment -> one with little or no oxygen: bottoms of lakes, swamps, and shallow seas. Hard blackish substance formed from organic material compressed under very high pressure.