Is there evidence of underground bases in Australia?

Australia has enormous underground infrastructure. Mount Isa, Olympic Dam, Kalgoorlie's Super Pit and hundreds of other mines run kilometres below the surface. Coober Pedy has an entire underground town with homes, churches and hotels, built to escape the heat.

Sydney and Melbourne sit above extensive rail, water and utility tunnels, including disused wartime shelters and abandoned stations that urban explorers document in detail.

Pine Gap, the Harold E. Holt naval communication station at Exmouth and various defence installations have restricted areas and underground components. Their existence is acknowledged; their internal operations are classified. That is ordinary for defence infrastructure in any country.

Classified is not the same as denied. A facility you can find on a map with a publicly stated purpose is not a hidden base.

Claims of vast secret cities beneath the Nullarbor require a supply chain: excavation contractors, spoil removal, ventilation, power, water, thousands of workers and a budget line. Every one of those leaves traces in procurement records, satellite imagery and employment histories.

The absence of any of those traces is the reason these claims stay claims.

Geoscience Australia publishes geological and borehole data. Mining tenements are public. Satellite imagery is freely available at high resolution. Freedom of information requests work, slowly. Heritage registers list decommissioned military sites.

Anyone genuinely curious has better tools available now than investigators had thirty years ago, and using them beats arguing from a blurry photograph.

The order's own first principle is light: pursue knowledge, but pursue it properly. Wanting something to be true is the most common reason for believing weak evidence, and recognising that impulse in yourself is most of the skill.