Mount Etna is shown belching out lava, April 24, 2012, as the volcano continues to erupt throughout the month of April, in Catania, Italy. Europe's tallest and most active volcano has the longest period of documented eruptions in the world. (Angela Platania/Zuma Press)

Election worker Khalid Battle reads a book as he waits for voters to cast their ballots in the Pennsylvania primary election at Memorial Gospel Crusades Church, April 24, 2012, in Philadelphia. Voters in New York, Connecticut, Delaware, Rhode Island and Pennsylvania were heading to the polls on Tuesday for primary elections. (Matt Rourke/AP Photo)

An Indian Hindu devotee swings her head in a trance-like state inside the Balaji Kama Kheda Temple, in Kama Kheda, Rajasthan, India, April 24, 2012. Many people visit the temple for specials blessings believed to help ward of evil spirits. (Kevin Frayer/AP Photo)

A new Boeing 787 Dreamliner, fitted with Rolls Royce engines, sits on the tarmac at Manchester Airport April 24, 2012 in Manchester, England. The Dreamliner is made of composite materials and uses 20 percent less fuel than equivalent aircraft. (Christopher Furlong/Getty Images)

The warship Vasa is seen at the Vasa Museum in Stockholm, Sweden, April 23, 2012. The Swedish warship built in the 17th century sank after sailing less than a nautical mile on its maiden voyage on Aug. 10, 1628. It was salvaged with a largely intact hull in 1961. (Xu Liang/Xinhua)

A member of a military honor guard adjusts the curtains in preparation for a welcoming ceremony for South Sudan's President Salva Kiir Mayardit hosted by Chinese President Hu Jintao in the Great Hall of the People in Beijing, April 24, 2012. Kiir's visit comes at a time when war between Sudan and South Sudan threatens to escalate. (Adrian Bradshaw/EPA)

Rapeseed blooms are shown in a field close to Stonehenge, April 24, 2012, near Amesbury, England. The vibrant yellow blossoms have become a familiar sight in rural areas of the United Kingdom and are now one of the most popular crops for farmers. (Matt Cardy/Getty Images)

A Pakistani child, whose family was displaced by 2010 floods from a village near Multan, sleeps in a hammock attached to a bed, in a makeshift tent on the outskirts of Islamabad, Pakistan, April 24, 2012. (Muhammed Muheisen/AP Photo)

President Barack Obama speaks at the University of North Carolina, April 24, 2012, in Chapel Hill, N.C.. President Obama spoke to the group about the potential of student loan interest rates doubling this summer. (Brendan Smialowski/AFP/Getty Images)