Passage: In 1897, Andrée's daring or foolhardy undertaking nourished Swedish patriotic pride and Swedish dreams of taking the scientific lead in the Arctic. The title of Engineer—Ingenjör Andrée—was generally and reverentially used in speaking of him, and expressed high esteem for the late 19th-century ideal of the engineer as a representative of social improvement through technological progress. The three explorers were fêted when they departed and mourned by the nation when they disappeared.

Question: Why was the nation mourning?
they disappeared