Rio de Janeiro's state military policemen aim their guns during a violent protest in a favela next to Copacabana, Rio de Janeiro on April 22, 2014. Violent protests broke out in Rio's landmark beachfront district, Copacabana, following the death of a resident last weekend during clashes with the Army in a nearby favela. (Photo by Christophe Simon/AFP Photo)

A man wearing a “Pinhead” mask walks through the grounds of St Mary's Church, Whitby, during the Whitby Gothic festival taking place this weekend, on April 27, 2014. The twice yearly event attracts Goths from across the UK and beyond to the historic fishing town and is a great boost to the local economy. (Photo by Anna Gowthorpe/PA Wire)

A tour group wanders through block 7 of Eastern State Penitentiary in Philadelphia, Pennsylvania April 30, 2014. Opened in 1829, with the original corrective system of “confinement in solitude with labor”, the penitentiary housed about 75,000 inmates in its 142 years of operation. At Eastern State reunions, former inmates, staff and guard gather to share memories and trade stories and get a chance to describe their experiences in question-and-answer sessions with the public. (Photo by Mark Makela/Reuters)

A soldier poses with a Hythe Mk III Gun Camera during training activities at Ellington Field, Houston, Texas in April of 1918. The Mk III, built to match the size, handling, and weight of a Lewis Gun, was used to train aerial gunners, recording a photograph when the trigger was pulled, for later review, when an instructor could coach trainees on better aiming strategies. (Photo by Harry Kidd/WWI Army Signal Corps Photograph Collection via The Atlantic)

The Hubble Space Telescope is shown following its release from the space shuttle Discovery Wednesday, February 19, 1997. The Hubble Space Telescope, one of NASA'S crowning glories, marks its 25th anniversary on Friday, April 24, 2015. With more than 1 million observations, including those of the farthest and oldest galaxies ever beholden by humanity, no man-made satellite has touched as many minds or hearts as Hubble. (Photo by AP Photo/NASA)

Descendants of American Southerners Wearing Confederate-era dresses and uniforms dance during a party to celebrate the 150th anniversary of the end of the American Civil War in Santa Barbara d'Oeste, Brazil, Sunday, April 26, 2015. For many of the residents of Santa Barbara d'Oeste and neighboring Americana, in Brazil's southeastern Sao Paulo state, having Confederate ancestry is a point of pride and is celebrated in high style at the annual “Festa dos Confederados”, or “Confederates Party” in Portuguese. (Photo by Andre Penner/AP Photo)

An employee of the National Park Service takes a selfie with President Barack Obama, left, in the background meeting with the crowd after a tour of Everglades National Park on Earth Day, Wednesday, April 22, 2015, in Florida. Obama used the visit to warn of the damage that climate change is already inflicting on the nation's environmental treasures. (Photo by Lynne Sladky/AP Photo)

In this April 23, 2015 photo, friends Jia Haixia, left, and Jia Wenqi work to plant a tree in a field in Yeli village near Shijiazhuang city in northern China's Hebei province. For the past 13 years, Jia Wenqi, who has no arms, and Jia Haixia, who is blind, have worked together to plant and water more than 12,000 trees near their village. (Photo by Helene Franchineau/AP Photo)