The world mourns the sudden loss of four-year-old Knut, Berlin’s most popular polar bear.  After being rejected by his mother, Knut was raised by zookeeper Thomas Dörflein at the Berlin Zoo.  There, he quickly became an international celebrity.   The craze surrounding Knut, dubbed “Knutmania,” gained worldwide attention as his miniature stuffed animals, DVDs and books sold out. Though experts are debating the exact cause of Knut’s death, all can certainly agree: Knut will be missed. Kaitlin Flynn, Global Animal

As a baby, Knut lies in an incubator at the Berlin Zoo. (Peter Griesbach/EPA)
Baby Knut could make anything into a toy. He's shown here at the Berlin Zoo on March 2, 2007. (Getty Images)
Knut rolls around in the sand. (Franka Bruns/AP)
Knut sticks close to keeper Doerflein during his first public appearance. (Herbert Knosowski / Associated Press)
Keeper Thomas Doerflein presents Knut at his first public appearance in the Berlin Zoo. (Franka Bruns/AP)
Knut explores his enclosure on April 3, 2007. (Andreas Rentz / Getty Images)
A reproduction of a special stamp honoring Knut in 2008. (Gero Breloer / European Pressphoto Agency)
Knut celebrates his second birthday with treats in 2008. (Michael Gottschalk / AFP/Getty Images)
Knut goes for a swim in the pool in his enclosure in July 2009. (David Gannon / AFP/Getty Images)
Knut rolls in the snow on Jan. 8, 2010. (John MacDougall / AFP/Getty Images)
Knut rests in the snow in his enclosure on Dec. 10, 2010. (Johannes Eisele / AFP/Getty Images)
Children stand next to flowers placed at the main entrance of the Berlin Zoo on Sunday, a day after Knut's death. (Tobias Schwarz / Reuters)
Flowers and a message to Knut and his late keeper near the famous bear's enclosure on Sunday. (Andreas Rentz / Getty Images)
Flowers, mementos and notes from fans are shown near Knut's enclosure on Sunday. Well-wishers flocked to the Berlin Zoo to pay tribute to Knut. (Tobias Schwarz / Reuters)