Knut, the Berlin Zoo's famously adorable polar bear, has died. Knut collapsed Saturday and was found floating in the water at the zoo's polar bear enclosure, which he shared with his mother Tosca and two other bears. Knut had recently turned 4 years old. The cause of death is unclear. "It's terrible," Berlin Mayor Klaus Wowereit told the Berlin newspaper BZ. "He had a special place in all of our hearts. He was the star of Berlin Zoo." Knut was rejected by his mother as a cub and raised by zoo keeper Thomas Dorflein. Dorflein died unexpectedly of a heart attack at the age of 44.