In a single pool, in a single Nelson cave lies a critically endangered creature with a curious and little-known history. Naomi Arnold finds about one of the Maitai's secret inhabitants that is found nowhere else in the world.

Consider this snail. It is see-through, blind, and about the size of a largish grain of sand. It lives in only one pool, in one cave, in one stream – Nelson's Maitai River.

At 1.7mm high and 1.2mm long, you'd be forgiven for overlooking it.

An enigmatic treasure, it was first discovered in the 1970s by Frank Climo, along with several other species. They were thought to be one; until in 2001, visiting Austrian scientist Martin Haase and research assistant Christina Mosimann worked on a NIWA project to untangle New Zealand's freshwater snails, which live mostly in springs and groundwater.

Haase, who had worked extensively with this group of snails in Europe, visited all sorts of places in New Zealand, basing his research on previous data and museum collections, and analysed the tiny life in the rivers and caves - though caving isn't quite his forte.

"I'm a little bit of a coward when it comes to caving so I don't go very deep," he says, half joking.

He identified many snails as different from their cousins, using more advanced methods of classification than were available to Climo at the time. So Nelson City got its very own snail.

Identifying them is relatively low-key. The pair went for the one and a half hour hike up the river to the Maitai Cave Once there, they picked up stones, washed them in trays, sieved the sand, and looked at what fell off. Then they took their snails out into the light and fixed them in preservative to take back to the lab. In 2008, Hadopyrgus ngataana was scientifically described and named (ngataana comes from Maori words for "cave" and "snail").

In 2013, it achieved the dubious distinction of being named "Nationally Critical" on the Department of Conservation's threatened species list, just one level above extinct. Haase's name appears 91 times on that list - he's been instrumental in discovering a host of our threatened freshwater invertebrates.

Sadly, ngataana is pegged to decline, as it occupies such a limited area, and the Maitai Caves are used recreationally.

From his office the University of Greifswald in Germany, Haase recalls his work in Nelson and New Zealand with fondness. He will return in March for further study, this time on the New Zealand mud snail, the most common of the New Zealand freshwater snails. (In case you're interested, the collective noun for a group of snails is "a walk", "a rout", or "an escargatoire").

"It is a beautiful country, and you know it's popular among Europeans for holidays; but it is attractive for many different reasons," he says. "There are still many pristine sites; and then for biologists, the unique species there. New Zealand is full of endemic species and the Nelson area is particularly rich."

Haase got into snails as a student in his home country of Austria, when he was looking for a subject for his thesis.

"One of the potential supervisors suggested I could get famous with this group of snails," he says, once again half joking.

"That was the wording. I had no idea until then how difficult that might be; what it means to go out in the field and find them and take them back to the lab and work on them. This group [of snails] has taken me around the globe so far and has contributed to a fairly interesting life as a scientist."

The majority of his snails are only 2mm-3mm tall, but they are extremely diverse, Haase says.

"The number of species in this group is very high. Another fascinating aspect is that many of the non-marine species have very narrow ranges; they are highly endemic where they occur. This holds for Europe, the American continents, Australia, New Zealand - basically everywhere. This makes them very interesting from an evolutionary point of view and for conservation."

Snails, admittedly not the most dynamic of creatures, might only occur in a single valley, spring, or cave, living there and nowhere else for thousands of years.

Haase was surprised, though, at the number of species he uncovered here.

"It was interesting and fun to discover," he says. "The general pattern is very similar wherever you go in the world, but still, in the more developed countries you assume that scientists have seen more or less everything. But this is definitely not the case in New Zealand. It was the first time I was there and I was naive and therefore also surprised that New Zealand still has so many things to be discovered. Scientists have not looked into every hole in New Zealand."

Many snails in New Zealand live out their lives undiscovered, and of those that are named and officially entered into the human record, not much is known about them.

It matters not just because a unique life form is gone forever from the planet but also because species that only live in certain habitats may act as canaries in a coal mine, he says.

"There are good indications that they need clean and clear spring water, and when quality deteriorates they may go as well," he says. "In Europe our spring snails are used to indicate water quality - several countries have this system. This is certainly an important function of species - any kind of species that are specialists of certain habitats are indicators of the functioning of this habitat.

"We can measure chemical and physical parameters, but these are only snapshots of the moment. When you find a population of snails or crustaceans, you know they are there for quite some time, certainly much longer than the physical and chemical conditions I encounter when taking my sample."

The latest on this family of snails will be released in a research paper out soon. Haase and his colleagues have reconstructed the relationships and biogeography of freshwater snails occurring in the world, and it has, he says, uncovered more differences among the world's freshwater snail population than previously realised. Those in New Zealand, Australia, and the Pacific are different from those in North America in Europe. There are even clues about how they spread in our part of the world. It turns out that Australian freshwater snails spread north to Papua New Guinea and New Caledonia. Fiji and Vanuatu's snails came from the Austral Islands, and because there are no marine relatives known to these islands, they're likely have a common ancestor in New Zealand – meaning it was our mighty snails that went on to conquer the Pacific.

One theory about how they got to those islands is that they flew, attached to the feathers of a bird, surely a terrifying and unlikely journey for a miniature snail.

Not so far-fetched, Haase says, even though no-one has seen it happening. Bird species common across the Pacific Islands include the Pacific black duck, waders, rails, and a cuckoo, which migrates to the Pacific.

"Many birds come to streams and springs to take baths, so they can get stuck in the plumage, and the birds must fly to a different spring to release them again. We've seen that on insects; I wouldn't think that insects could carry them across 3000 km of open ocean, but certainly within New Zealand."

He explains that snails have a "door" they can close on their shell, and a leg of an insect can get trapped in it - and then the snail is attached, unaware that it's about to take the ride of its life. Insects with clams and snails attached have been found.

"It's most likely how they get around."

But because many of the snail species occur in one place only, if you destroy or disturb that area, you could wipe an entire species off the face of the earth.

"We have known places where they covered the spring, or put a house on it or turned it into a fountain - and all the life was gone."