A rebel fighter carries his weapon as he moves through a hole in a wall in the old city of Aleppo near the frontline against forces loyal to Syria's President Bashar al-Assad December 28, 2014. (Photo by Jalal Al-Mamo/Reuters)

Participants climb over a wall at the “Tough Mudder” endurance event series in Arnsberg September 6, 2014. Competitors who took part in the endurance event saw themselves having to overcome various military style obstacles. (Photo by Ina Fassbender/Reuters)

Opera singers perform in the pool “AquAria PALAOA” during a rehearsal at the swimming pool Neukoelln on April 27, 2011 in Berlin, Germany. The premiere of the underwater opera AquAria PALAOA is on May 1st. (Photo by Andreas Rentz/Getty Images)

Street fighting in Berlin between Government troops and Spartacists, during the Spartacist uprising which followed Germany's defeat in World War I. (Photo by Keystone/Getty Images). 1919

Egyptian President Hosni Mubarak speaks to the media after talks with German Chancellor Angela Merkel at the Chancellery on April 23, 2008 in Berlin, Germany. (Photo by Sean Gallup/Getty Images)

In the middle of the lobby of the Radisson SAS Hotel in Berlin stands a AquaDom standing 25 meters tall and is the largest cylindrical aquarium ever built. It’s filled with about 900,000 liters of seawater and has 2600 fish of 56 species.

Keith Richards, Mick Jagger, Ron Wood of the Rolling Stones perform during their "14 on Fire" show at Waldbuehne in Berlin, June 10, 2014. REUTERS/Thomas Peter

Dancers of the Friedrichstadt-Palast from the show “THE WYLD” pose during a promotional photocall in front of the Brandenburg Gate in Berlin, Germany, June 25, 2015. The 10.6 million euro ($13.5 million) show is the largest production budget in the 95-year history of Friedrichstadt-Palast. (Photo by Hannibal Hanschke/Reuters)