The Solar Impulse 2 airplane, flown by test pilot Markus Scherdel, flies off the coast of Oahu during a test flight from Kalaealoa Airfield in Kapolei, Hawaii, March 3, 2016. The solar-powered plane is ready to continue its revolutionary voyage around the world. After the craft was grounded in Hawaii last July due to overheated batteries, it was repaired and upgraded, and a recent initial test flight proved successful. (Photo by Eugene Tanner/Reuters)

Beneath the Swedish capital lies an intricate web of underground train lines. More than 90 of the 100 stations in the 110km tunnel system, sometimes referred to as “the world’s longest art gallery”, have been decorated with paintings, installations, mosaics and sculptures by 150 artists since the 1950s. After spending a couple of weeks exploring arctic Norway and Sweden, London-based travel photographer Conor MacNeill headed underground to capture images of the metro stations. Here: A rainbow arcs over a girl on the platform of Stadion station. (Photo by Conor MacNeill/The Observer)

Chris Staring photographs a mysterious train graveyard in the heart of southern Bolivia, where the skeletons of British steam locomotives and rail cars rust away on the edge of the world’s largest salt flats. More than 100 rail cars and locomotives can be found in different states of decay in the train graveyard. (Photo by Chris Staring/Rex Features/Shutterstock)

A hyena eyes a herd of zebra at Lake Nakuru National Park, Kenya, August 19, 2015. The Park is home to some of the world's most majestic wildlife including lions, rhinos, zebras and flamingos. The scenery is stunning, from forests of acacia trees to animals congregating at the shores to drink. UNESCO says that with rapid population growth nearby, the area is under "considerable threat from surrounding pressures," particularly deforestation, a contributing factor in floods. (Photo by Joe Penney/Reuters)

A Russian daredevil has captured a vertigo-inducing selfie – while standing on top of a Dubai skyscraper. Nineteen-year-old Alexander Remnev scaled the Princess Tower – the worlds tallest residential building at 1,350ft – before getting his camera out to take these stomach-churning pictures. Photo: He leans on the very top of the tower as he takes this vertigo-inducing picture. (Photo by Alexander Remnev/Caters News)

Despite any political differences between the United States and Russia, the space agencies of the two countries continue their cooperative work in Earth's orbit, aboard the International Space Station. Apart from the research being done in microgravity, ISS crew members continue to send back amazing images of our home world, photographed from low Earth orbit. Gathered here are recent images of Earth from aboard the ISS, and from a handful of other NASA satellites.

These pictures show a five-year-old boy who is already extremely comfortable around alligators. Blaise's dad T-Mike runs the Kliebert Gator Farm, the biggest and oldest in the world and which was started by his grandfather Harvey in 1957. Photo: T-Mike Kliebert and his son Blaise tackle Chinaman a 12 foot alligator in Hammond, Louisiana. (Photo by Barcroft Media)

An Edinburgh Fringe performer shows off his makeup on the Royal Mile, in the final weekend of this year's festival. The largest performing arts festival in the world has been running for the past three weeks and has enjoyed an increase in venues and visitors compared with previous years. (Photo by Jeff J. Mitchell/Getty Images)