Researchers ranked countries based on 25 indicators, including water and air quality, greenhouse gas emissions and the impact of the environment on the health of the population. A score of 100 is excellent. Sierra Leone ranks at the bottom of the list with a score of 32. The U.S. ranks in the middle of the pack with 63.5. Iceland took top honors with a score of 93.5 thanks to ample clean water, lots of protected nature areas, good national health care and a plenitude of usually clean geothermal power.