Ants are often a target of tropical biodiversity assessment because of their ecological dominance and value as indicators of ecosystem health, but high microhabitat specificity, patchy distribution and cryptic habits of many species make effective sampling problematic. Although tropical ant faunas have long been known to show strong vertical stratification, only recently has it been recognised that this can include a high diversity of subterranean species that are poorly sampled using traditional methods. Global diversity patterns of subterranean ants and their responses to above-ground disturbance remain largely unknown. We describe ants collected in 360 subterranean traps distributed across 15 sites representing contrasting soil types (volcano-sedimentary and ultramafic) in New Caledonia, a recognised Global Biodiversity hotspot. New Caledonia has a diverse above-ground ant fauna that includes spectacular radiations of both Gondwanan and Indo-Malayan genera from all above-ground layers (litter, epigaeic and arboreal), and so it might be expected also to harbour a subterranean ant fauna of high biogeographic and conservation significance. We show that New Caledonia supports an extremely depauperate subterranean ant fauna, especially on ultramafic soils, with only two cryptobiotic species recorded in subterranean traps, and a trap success rate up to an order of magnitude lower than in comparable Australian studies. Our results show that there is an uncoupling of ant diversity above- and below- ground, such that a high diversity and conservation significance of ants above ground is not necessarily matched below.