To the surprise of the global subterranean biology community, Bill discovered a completely new, remarkably diverse, and large fauna in a most unexpected place, namely the subterranean waters of Australia’s arid heart. Rather than restricting himself to a particular group of the fauna, Bill set about, in collaboration with national and international taxonomic experts, to systematically document the taxonomy of this entire fauna and, importantly, to establish its scientific and conservation importance through detailed and cutting-edge evolutionary research.

Bill and his collaborators made some of the most important taxonomic discoveries in Australia (Fig. 2). Some examples of these include the discovery of: i) a significant cave fauna comprising 33 troglobiotic species in 26 genera, in the Cape Range and Barrow Island (Western Australia: WA) terrestrial cave faunas (Humphreys 1993a, 2000); ii) a stygobiotic fauna in the Cape Range WA, comprising 30 species in 20 genera, which included two fish, and ancient crustacean lineages, such as the Class Remipedia and Order Thermosbaenacea, unknown previously from the southern hemisphere (Poore and Humphreys 1992; Humphreys 1993b; Yager and Humphreys 1996; Jaume and Humphreys 2001; Jaume et al. 2001; Wilson and Humphreys 2001); iii) the crustacean Order Spelaeogriphaea in the Pilbara WA, known previously from only two caves in South Africa and Brazil (Poore and Humphreys 1998); iv) a rich stygofauna in calcrete aquifers of the Pilbara, comprising both annelids – phreodrilid oligochaetes (Pinder and Brinkhurst 1997), and crustaceans – amphipods, copepods, ostracods, phreatoicid isopods, and syncarids (Humphreys 2001; King et al. 2022); v) an extraordinarily diverse fauna in the calcrete aquifers of central WA, comprising dytiscid diving beetles, currently with ~100 species identified (Watts and Humphreys 2009 and references therein), and crustacean species – amphipods (Cooper et al. 2007; King et al. 2012), copepods (Karanovic 2004; Karanovic and Cooper 2011), isopods (Taiti and Humphreys 2001; Cooper et al. 2008), ostracods (Karanovic and Marmonier 2002), and parabathynellid syncarids (Cho 2005; Guzik et al. 2008) and vi) significant subterranean faunas in Christmas Island anchialine systems (Humphreys and Eberhard 2001; Humphreys and Danielopol 2006; Kornicker et al. 2006), Kimberley WA freshwater karst (Humphreys 1999) and Ngalia Basin calcrete aquifers (Northern Territory) (Cho et al. 2006; Watts and Humphreys 2009; Watts et al. 2023). The Kimberley study included the discovery and identification of bathynellid syncarids (Cho et al. 2005) and phreatoicid isopod species (Wilson 2008).

Bill’s taxonomic and evolutionary research led to two major findings on the diversity and origins of the subterranean fauna of Australia. First, that the relictual nature of these faunas is emphasised by the evolutionary relationships of some taxa: Kimberley, Pilbara, Cape Range and Christmas Island caves contain species whose nearest relatives occurred in geographically remote caves (e.g. either side of the North Atlantic, Gondwanan fragments, or isolated seamounts), often indicative of a connection with the ancient Tethys Ocean. Cave fauna in the now arid Cape Range comprise a relict wet-forest litter fauna with both temperate and tropical affinities (Humphreys 1999, 2000). Detailed phylogenetic and population genetic analyses have revealed numerous instances of the colonisation of subterranean habitats from surface dwelling ancestors. These include the very diverse dytiscid water beetle fauna, which has also showed novel modes of speciation, including the possibility of sympatric speciation through ecological diversification in the calcrete ecosystems (Leys et al. 2003; Leijs et al. 2012; Langille et al. 2021). These studies also confirmed the original hypothesis of Humphreys (2001) that the numerous calcrete aquifers in the Australian arid zone each contain distinct short-range endemic species; essentially, they act like islands under the desert (Cooper et al. 2002).

In summary, Bill identified a major gap in Australia’s taxonomic knowledge following his discoveries of unique subterranean ecosystems in Western Australia, and his research made significant progress towards founding the field and addressing this gap. Further, he established a global network of collaborators (taxonomists and molecular systematists) who have since worked toward documenting the faunal diversity, its origins and evolution using both morphological and molecular methods. This research continues today. Bill also fostered and mentored the research program of early career researchers in other parts of Australia leading to discoveries of significant diversity in groundwater or cave fauna in New South Wales (investigator: Peter Hancock), South Australia (investigator: Remko Leijs) and Tasmania (investigator: Stefan Eberhard).

In addition to significant taxonomic research, Bill always sought to improve understanding of the ecology of groundwater (Humphreys et al. 2006) and cave ecosystems (Humphreys and Feinberg 1995), and the biology of their fauna (e.g., thermal tolerance of diving beetles, Jones et al. 2021). Much of his recent research has focused on the anchialine ecosystem in Cape Range peninsula and Barrow Island (Humphreys 2000), and a calcrete aquifer at Sturt Meadows in Western Australia (Allford et al. (2008)), where he and colleagues established a valuable series of groundwater monitoring bores. The multidisciplinary work carried out at Sturt Meadows has consistently improved our understanding of carbon flows (Saccò et al. 2020, 2022), food web interactions (Saccò et al. 2019) and rainfall-driven cascade effects (Saccò et al. 2021) in calcrete aquifers. At Cape Range he led multiple fieldwork campaigns to investigate the ecological functioning of the Bundera Sinkhole, the only continental anchialine system known in the Southern Hemisphere, resulting in multiple recent publications on the diversity (Elbourne et al. 2022) and functions of the resident microbial communities (Ghaly et al. 2023). At Barrow Island, his research significantly contributed to the understanding of ecological patterns shaping hypogean fauna (Humphreys 2000), triggering enhanced protection for endangered subterranean biota.

Many of the groundwater ecosystems identified by Bill are in regions of considerable significance to the mining industry (e.g. Pilbara and Goldfields regions of central WA), and where groundwater is a major source of water for these operations, as well as for pastoral and domestic consumption. Furthermore, calcrete, the major limestone aquifer matrix for these faunas, is used extensively as a neutralising resource, for mineral processing and road construction. The discovery of a fauna within individual aquifers that comprised endemic species with very small distributional ranges, many of which represent ancient lineages previously unknown in Australia, has had major conservation and management implications at local and national levels.

Bill widely disseminated his research findings in reports (e.g. 2006 State of the Environment Report on Emerging Issues in Groundwater Fauna) and seminars to raise awareness amongst regulatory and management authorities of the significance of these faunas and this led directly to policy developments to protect groundwater fauna across Australia. Bill’s research provided critical background information for environmental assessments and spawned considerable work in the environmental consultancy industry as regulatory authorities accepted that the high biodiversity and short-range endemism of the fauna had significant conservation implications. For example, in WA, mining companies are required currently by the Environment Protection Agency to “identify stygofaunas in the vicinity of proposed mine sites, identify any effects of mining operations, and prepare an action plan to conserve the fauna if results indicate that a fauna could be adversely affected by mining” (Playford 2001, Report to the Environment Protection Agency, WA; Guidance Statement 54, EPA 2003, and 54a, 2007). The impact of his research and advocacy in this space will help protect subterranean fauna in Australia for generations to come.

Since the 1990s, Bill has been an active member of the subterranean biology community, including as an editor and member of the editorial board of Subterranean Biology, and a member of the council for the International Society of Subterranean Biologists (ISSB). He was vice president of the society from 2004 to 2008 and 2012 to 2014, and a regular member of International Scientific Committees for ISSB conferences since 2010. He was also convenor for the 19^th International Conference on Subterranean Biology (ICSB) in Fremantle, Western Australia, which proved to be a highly successful and profitable meeting, as well as initiator and co-convenor of conferences on anchialine ecosystems. He regularly attended all the ICSB conferences, where he gave multiple invited plenaries, most recently attending the ICSB in Cagliari, Sardinia, Italy where he was awarded an Honorary Membership of ISSB. He will be sadly missed at the 27^th ICSB in Romania. Other impacts at the international level include his membership of the IUCN Species Survival Commission – Cave Invertebrate Specialist group for the last 12 years.

In Australia, Bill was awarded the prestigious Eureka Prize for excellence in taxonomy in 2008, and the Ivor Beatty Prize from the journal Pacific Conservation Biology in 2020. Bill’s conservation impact included nine years as a member of the Australian government’s Threatened Species Advisory committee (2006–2014), and ongoing membership of the Inaugural Ningaloo Coast World Heritage Advisory Committee (2012–2026). He was also a member of the Subterranean Fauna Technical group for the WA Environmental Protection Authority, among other WA state government biodiversity committees.

Bill has impacted and inspired numerous colleagues and students throughout the world, as evidenced by the outpouring of email comments following the notification of his death. He was always generous with his advice and he developed many long-lasting friendships in the international subterranean biology community.

For those of us lucky enough to join Bill in the field, it was at times like travelling with Indiana Jones, as he turned from a highly intellectual gentleman research scientist to a practical Australian bushman, often seen to be climbing down dilapidated wells, ropes tied to crusty old windmills, fossicking around in the dirt looking for elusive mineral exploration bores, and rescuing highly venomous brown snakes and goannas from old town bores (Figs 3, 4). His knowledge of subterranean fauna, and speed sorting the day’s stygofauna catch, glass of red wine nearby, were legendary. Bill passed away after a brave, but short battle with mesothelioma. He is survived by the wonderful and always supportive Rae, their two beautiful daughters, Miranda and Phoebe and partners, and two recently born grandchildren, Jasper and Margot, who we all wish will rejoice in Australia’s varied ecosystems as much as Bill did.