Using a porthole camera the architecture of the warrens of the southern hairy-nosed wombat Lasiorhinus latifrons was examined at two sites in southern Australia. The complexity of warrens varied from simple, single-entrance warrens to a system 89 m long with 28 entrances. Logging of deep tunnel temperatures showed daily ranges of < 1 °C over periods in which surface temperatures ranged up to 24 °C, but mean tunnel temperatures varied 15.9 °C between winter and summer. Humidity gradients were maintained between the tunnel and the surface and depended on the length and depth of the tunnel. Oxygen and carbon dioxide levels were similar to those in free atmospheric air in unoccupied tunnels, but in occupied tunnels reached 16.3% and 3%, respectively. There is no specific orientation of tunnel entrances or defined mound shape. Wombat warrens ameliorate surface conditions in habitats of low humidity and high temperatures while humidity advantages promote the construction of warrens beyond the requirements for temperature modulation.