The Underworld 2.9 release is available from Github, as a docker container and via zenodo (doi:10.5281/zenodo.3964957) it is also available through pip install for the first time (see below). Underworld paper in the Journal of Open Source Software The paper of record for underworld 2 has been published. It does tie to the Underworld 2.9 release but is sufficiently broad to cite for all 2.x releases.