Avatar: The Last Airbender Season 2 Episode 10 : The Library
At an oasis, the group encounters a professor who tells them about a hidden Spirit Library in the desert. Inside, Sokka discovers a crucial weakness to the Fire Nation that could end the war: the date of the upcoming solar eclipse which will cripple the Firebenders' Firebending. The spirit of the library, Wan Shi Tong, refuses to allow them to leave with the knowledge and sinks the library into the sand. Meanwhile, Appa is kidnapped by Sandbenders. They all escape from the library but are devastated by the loss of Appa.