Pumpjacks at the Inglewood oil fields in California in March. Some of the most controversial methods of oil extraction, like fracking, oil sands production and Arctic drilling, are also expensive. That's made them less profitable as the price of oil continues to fall. Richard Vogel/AP hide caption

itoggle caption Richard Vogel/AP

The Erebus and the Terror among icebergs, as illustrated in The Polar World by G. Hartwig in 1874. Sir John Franklin, British naval officer and arctic explorer, commanded the 1845 expedition of the ships to search for the Northwest Passage. All members of the expedition perished. G. Hartwig/Universal History Archive/Getty Images hide caption

itoggle caption G. Hartwig/Universal History Archive/Getty Images