Marble Canyon, Grand Canyon, Arizona, U.S.

Down on the mile-deep floor of the Grand Canyon, the stillness allows the subtlest natural sounds to emerge, from the call of a peregrine falcon overhead to the scamper of a scorpion underfoot. Rock layers tell their own story, revealing nearly two billion years of geology. But the serenity is no longer guaranteed. It is frequently broken by air tours. In 1999, Senator John McCain of Arizona introduced a law that helped cut down on this persistent source of human noise. But up to 400 flights still cross the canyon or fly below the rim each day.