Purdue University students are testing new software that may track and warn about tornadoes, such as this one which struck Rochelle, Ill., in early April. Walker Ashley/AP hide caption

itoggle caption Walker Ashley/AP

A boat collects oil that leaked from the Deepwater Horizon wellhead in the Gulf of Mexico in April 2010 near New Orleans. Chris Graythen/Getty Images hide caption

itoggle caption Chris Graythen/Getty Images

A tray of sardines in Costa Mesa, California, in this November 17, 2014 photo. Plummeting sardine populations force a complete ban on sardine fishing off the U.S. West Coast for more than a year. LUCY NICHOLSON/Reuters /Landov hide caption

itoggle caption LUCY NICHOLSON/Reuters /Landov

At the Pelican Hill golf resort in Newport Coast, Calif., water conservation is an obsession. Yuki Shimazu/Flickr hide caption

itoggle caption Yuki Shimazu/Flickr

Workers pick asparagus in early April at Del Bosque Farms in Firebaugh, Calif. This year, some farmers in the state will get water, others won't, based on when their land was first irrigated. David Paul Morris/Bloomberg/Getty Images hide caption

itoggle caption David Paul Morris/Bloomberg/Getty Images

Students at the Pennsylvania College of Technology are learning a technique called "tripping pipe," moving a pipe from a stack into a horizontal position and lowering it down into a well. The students train on a practice drilling rig to learn how to be roustabouts. Jeff Brady/NPR hide caption

itoggle caption Jeff Brady/NPR

An undated file photo provided by the Wisconsin Department of Natural Resources of a northern long-eared bat. A fungal disease has devastated the species, now listed as threatened. AP hide caption

itoggle caption AP

Poland's second-largest city is also a major tourist destination. Krakow (seen here at night from the Krakus Mound) is suffering some of the worst air pollution in Europe. Arek Olek/Flickr hide caption

itoggle caption Arek Olek/Flickr

The Mariposa border crossing, as seen from Nogales, Ariz., September 2013. This land port serves as the main point of entry into the U.S. for fresh produce from Mexico. A lot of that produce gets rejected just past the border, even though it's perfectly tasty and edible. David Kadlubowski/Corbis hide caption

itoggle caption David Kadlubowski/Corbis