Most ants live underground, but that doesn’t mean they spend much time in direct contact with the soil. Some nests are worth a detailed look, as not all tunnels through the dirt are as simple as they may first appear:
This Australian Polyrhachis (Campomyrma) has lined its galleries with a fine wood pulp. I took this photograph last month in southern Australia. In the field, I was far too focused on the ants and their larvae to notice the carton substrate. At least, not right away. But once aware of it, I saw that every nest I uncovered had the pulp wallpaper.
This unusual use of organic matter likely provides the benefit of being better insulated and less prone to flooding or drying out than bare soil. The structure may also be a type of carton, meaning it also holds a connective fungus, but I was unable to find any published literature confirming it. In any case, ant nests themselves can be just as surprising as their hosts.
additional reading: Robson, S. K., & Kohout, R. J. (2007). A review of the nesting habits and socioecology of the ant genus Polyrhachis Fr. Smith. Asian Myrmecology, 1, 81-99.