1909. Facing nearly due west from ridge northeast of Como Lake. Light selection cut in open ponderosa pine. Ground cover is comprised of perennial grasses and forbs, including basalmroot. A few low-growing bitterbrush plants can be seen in the vicinity of horses and in distance on left. A group of willows can be seen behind horsemen at left center. Photo 87357/U.S. Forest Service hide caption

toggle caption Photo 87357/U.S. Forest Service

A scientist stands in front of an ice-rich permafrost exposure in the coastal zone of Herschel Island, Yukon Territory, Canada. Ice in the permafrost is melting, contributing to rapid erosion. Michael Fritz/Alfred Wegener Institute hide caption

toggle caption Michael Fritz/Alfred Wegener Institute

Steve Jobs and Steve Wozniak formed Apple Computer Co. on April Fool's Day, 1976. The Apple I, introduced soon after, was the first computer to come with a fully assembled motherboard (central printed circuit board). It sold for $666.66. Science & Society Picture Library / Contributor/Life hide caption

toggle caption Science & Society Picture Library / Contributor/Life

The facilities at the Fukushima Dai-ichi nuclear power plant were designed to withstand strong earthquakes and tsunamis, but not to the strength and size experienced on March 11. Air Photo Service/AP hide caption

toggle caption Air Photo Service/AP

A Tokyo Electric Power Co. worker looks at gauges in the control room for Units 1 and 2 at the Fukushima Dai-ichi plant, Wednesday. AP Photo/Nuclear and Industrial Safety Agency hide caption

toggle caption AP Photo/Nuclear and Industrial Safety Agency

A fallout shelter sign graces the Madison County Courthouse in Huntsville, Ala., in 2007. The county is working on a plan to identify shelters that can house up to 300,000 people in the event of a nuclear incident. Dave Martin/AP hide caption

toggle caption Dave Martin/AP