In the late 1910s, Under was involved in the influential Siuru literary movement. Named after a fire-bird in Finno-Ugrian mythology, it was founded in 1917 and was an expressionistic and neo-romantic movement that ran counter to the Young Estonia formalist tradition. Other Estonian poets and authors involved in the Siuru movement at the time were Peet Aren, Otto Krusten, Friedebert Tuglas, Artur Adson, August Gailit, Johannes Semper and Henrik Visnapuu. Between 1917 and 1919, the group of poets published three volumes of poetry. In 1919 conflicts within the group led Visnapuu and Gailit to leave, while Johannes Barbarus and August Alle joined as new members.