The long-awaited panda cub born Sept. 16 at the National Zoo in Washington has died, reports the AP. Zookeepers found the cub dead this morning after hearing distressed sounds from mom Mei Xiang. A cause of death is yet unknown, though the cub appeared outwardly to be in good condition. Panda cubs, born the size of a stick of butter and every bit as helpless, face a perilous first few weeks of life with threats ranging from infections to accidentally being crushed by their mothers. Mei Xiang has thus far seen one sole cub, Tai Shan, reach adulthood.