Oil shale and tar sands are other sources of liquid fuel that are harder on the environment than petroleum.

NPR: Will Pricey Fuel Prompt Greener Energy Habits?

They will soon bioengineer bacteria to melt oil out of tar sands, turn grass into diesel fuel and scavenge natural resources of every kind out of low-grade, thinly dispersed deposits.

FORBES: Techno-Optimism

Tar sands look like dirt and smell like diesel fuel.

NEWYORKER: Unconventional Crude