The cave amphipod Norcapensis mandibulis, an endemic genus known from  only four caves at an altitude of about 200 m in Cape Range, WA.

The cave amphipod norcapensis mandibulis, an endemic genus known from only four caves at an altitude of about 200 metres in Cape Range, Western Australia. Photo: Bill Humphreys

Beneath the very feet of Australians lives a mysterious group of animals known as stygofauna. Named after the River Styx, where dead souls cross from Earth to Hades in Greek mythology, these little creatures dwell in perpetual darkness, in groundwater, and were long thought to be rare in Australia. Only in the last two decades has the startling richness and strangeness of this subterranean life come to light.

''It's almost like when Captain Cook first reached our shores and saw kangaroos,'' say professors Craig Simmons and Peter Cook, of the National Centre for Groundwater Research and Training. ''We're aware that these strange creatures exist but we are discovering new ones all the time, and we still know little of what they do.''

Simmons says: ''These little Australians are really just becoming known to science, and they continually surprise us with their diversity and quaint characteristics. So far as the public is concerned, we've barely begun to explore this fascinating underground world of curious creatures.''

Most of our stygofauna are crustaceans, but they also include worms, gastropods, beetles, mites and fish. Some happen to live in groundwater by chance, some are temporary dwellers, but most can't survive outside the underworld.

They typically have little or no sight but have other senses enhanced to guide them in a lightless world. Most are colourless - which gives them a translucent look - and have few hardened body parts. The larger species swim through micro-caves, while smaller species, often with long, thin bodies, move between sediment particles in aquifers. They have no circadian rhythms to clock the time of day, as they never see the sun. As a result, compared with their surface equivalents, they grow slowly, have few young, live long lives and stay very close to home. Their slow lifestyle is regarded as an adaptation to the low-energy environment in which they live.

''Two decades ago, Australia was considered to have a rather uninteresting subterranean fauna'' says Professor Bill Humphreys, a senior curator at the Western Australian Museum. ''This was thought to be due to the relative lack of moisture and limestone karst resulting in few caves where they are usually found.'' (Karst is a surface formed by the dissolving of soluble rocks like limestone and dolomite.)

In a quest to explore this assumption, Humphreys and colleagues Steven Cooper and Andy Austin of the University of Adelaide and the South Australian Museum and other ''stygobiologists'' - as these explorers of the lightless realms are known - have been dropping fishing lines (with sampling nets attached) into little holes in the desert for the past 15 years.

In that time, they have discovered at least 750 species entirely new to science, mainly in the Kimberley and Pilbara regions of north-western West Australia, the Yilgarn region of WA, the Nullarbor region and South Australia. As a result of their work, Australia is now regarded as one of the world's hotspots for subterranean biodiversity, with an estimated 4000 different species. ''They have different backgrounds, most are native - and exclusive - to Australia,'' Humphreys says. ''Some are from extremely old lineages, with ancestors dating back to Gondwana and Pangaea or the Tethys Ocean, 200 million years ago. Some display a close relationship with species from other continents which indicates that their ancestors came from a time before the break-up of the supercontinents. Compared with these primordial creatures, the diving beetle that went underground and adapted to the subterranean life eight to 15 million years ago is considered to be a recent coloniser of the underground.''

Scientists have also discovered that stygobites - stygofauna species that are permanently confined to the underworld - are usually restricted to a single aquifer, he says.

''They are found in many types of geological formation, anywhere there are interconnected voids such as karst limestone, fractures and gravels. Most of these groups of creatures occur in small patches and might look and behave similarly but are actually different species. A community of stygobites found at one location could comprise a group of different species from another patch just one kilometre away. This means that a lot of these species are extremely rare and localised.

''The diverse community we have uncovered so far indicates that there is an entire ecosystem in the underworld made up of stygofauna and microbial populations or biofilms. As these creatures depend on groundwater to survive, they may be vulnerable to even small changes in their living environment.''

After spending two decades studying hyporheic zones, the shallow aquifers in riverbeds and banks where river water and groundwater exchange, Professor Andrew Boulton from The University of New England continues to be intrigued by the possible role of stygofauna in aquifers.

''Down in the hyporheic zone below the stream, you'll find various stygofauna as well as bugs from surface water that are small enough to penetrate the riverbed,'' Boulton says. ''The rich troves of biodiversity found here are often mind-blowing, and we thought they must play an important role in the aquatic ecosystem.''

Boulton and his research colleagues have suggested that the various activities of stygofauna may contribute to groundwater quality and interstitial flow in several ways.

''For example, when surface water sinks into the hyporheic zone, it carries dissolved oxygen and organic matter such as leaf fragments,'' he says. ''What the hyporheic stygofauna and other bugs do is break down the organic matter and free up carbon and other nutrients for the biofilms of microbes that coat the sediment particles within the aquifer. ''These microbial biofilms are crucial because they biologically filter the groundwater, functioning like a sandbed filter in a fish tank. The hyporheic stygofauna feed on the biofilms and this grazing activity stimulates growth and production of new biofilms which, in turn, soak up more nutrients. In fine sediments, stygofauna burrow and wriggle through the spaces as they feed. They produce mucus-coated faecal pellets and this, plus the burrows, can increase the porosity and permeability of the streambed aquifer.

''This creates a win-win scenario in the hyporheic zone - it improves water exchange and thus provides more oxygen and food for the stygofauna and, in turn, the microbes.''

Groundwater upwelling from the hyporheic zone into the stream also can benefit the surface ecosystem, Boulton explains: ''It is often enriched in particular nutrients by the microbial activities and can stimulate algal growth in 'hot spots' of upwelling water in streams and rivers. This algal production enters the surface stream foodweb, completing the recycling of nutrients in the surface water-groundwater ecosystem. We are starting to realise that the ecological health of many rivers relies on their exchanges with groundwater, and this means that the groundwater needs to be in good health too. In the same way that we use stream bugs as a measure of river health, perhaps we can use stygofauna to indicate groundwater health. This possibility is supported by some cases where no stygofauna were found in poor quality groundwater.''

While the exact roles of stygofauna remain poorly understood, there is a growing concern about the future of these unseen creatures as more and more Australian aquifers become contaminated by toxic hydrocarbons, pesticides and industrial runoff resulting from human activity, Boulton says.

''Often when groundwater is polluted, it goes undetected for a long while,'' he says. ''Consequently, stygofauna and microbial communities could go extinct long before we realise there is a problem. Remove them and we may be removing a crucial part of the biological filtration system of an aquifer, impairing its ability to cleanse groundwater naturally.

''Mining or water pumping activities may also harm these creatures, as no stygofauna have been shown to tolerate dehydration. In fact, they probably live underground in Australia because it is one place in an arid continent where water has persisted reliably for a long time.''

Simmons says: ''One thing that we do know about stygofauna is that they are not very mobile, so they make poor colonisers. This means that if all the creatures in one locality are wiped out, it is unlikely that others will quickly replace them. For a century, we have been pumping groundwater to drink, to water crops and for other uses. We have paid little attention to what happens to the aquifers and their living animals, with no realisation that pumping activities may actually destroy the creatures' habitat.

''Groundwater currently makes up about 97 per cent of all the available fresh water on the planet and accounts for about 40 per cent of humanity's total water supply. It provides drinking water to cities, is needed to grow much of our food and sustains many industries - yet in so many places across the globe, there is clear evidence that water tables are falling. This means humanity is extracting groundwater much faster than it is naturally replaced.

''Research around Australia points to stygofauna playing an important role in maintaining or indicating groundwater health. Their close ties with water quality indicate that they could play a part in the economic and ecological fate of Australians. We should pay attention to how our actions affect them, and in turn, to our nation's most precious resource - its enormous reserves of underground water.''

While stygofauna are out-of-sight, out-of-mind for most people, these little Australians continue to play their mysterious roles in the rich story of life on this amazing continent.

■ Mandy Thoo has her MSc in Science Communication. She works for Julian Cribb and Associates and is chief sub-editor on Science Alert. sciencealert.com.au.