Top 10 Things You Didn't Know About National Parks

Growing More Than Sequoias
National parks have been coping with a spate of unwelcome visitors recently — marijuana growers, who raise thousands of plants on federal lands. Usually tied to Mexican drug cartels, the pot plantations inflict considerable environmental damage and pose serious safety risks: they're frequently guarded by traffickers with automatic weapons and night-vision goggles (park rangers often arm themselves with M-16s as they scout for drug fields). In August, authorities at California's Sequoia National Park found a $36 million marijuana-cultivation operation just half a mile from a popular tourist site.

• Ken Burns, American Filmmaker