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It is widely recognised that arid Australia (including semi- and seasonally arid regions) supports an extremely diverse ant fauna. However, the truly remarkable extent of its diversity is not well appreciated, even among myrmecologists, and the reasons for such diversity are poorly understood. The vast majority of ant species in arid Australia are undescribed, and I argue that a recognition of the true diversity of the fauna would re-define the conventional understanding of global ant diversity patterns. For example, Monomorium Mayr and Tetramorium Mayr are formally recognised as widespread but essentially Afrotropical genera, whereas the large majority of species actually occur in arid Australia. The Australian fauna includes many genera each with > 100 arid-adapted species, and several of these have > 300 species. Melophorus Lubbock is likely to have well over 1000 species, making it one of the world’s richest ant genera despite being endemic to Australia. I estimate that the total size of the ant fauna of arid Australia is as much as 7500 species, which would make it rival, if not surpass, that of the Amazon basin. I argue that the phenomenal ant diversity of arid Australia is a product of its unique history of aridity and associated patterns of speciation and extinction over the past few million years. I propose that the Pleistocene glaciations that caused waves of mass extinctions throughout the northern hemisphere actually promoted speciation in Australia. Australia remained relatively warm during glacial periods, but there was massive movement of sand during these times of peak aridity, with up to 85% of the continent covered by desert dunes. This would have resulted in a highly fragmented biota confined to a very large number of isolated refugia, providing the perfect storm for speciation.