Hundreds of metres underground, behind a water-filled cave chamber, in some of the earth's silent, inhospitable holes, is where Ms McKinnon, 62, said she keeps life interesting.

Cave or sump diving is basically diving in dark, water-filled caves using air tanks and other equipment.

Overseas and across Australia it might involve cavern diving in wide, expansive water-filled spaces, where light is still visible.

In Tasmania it is something entirely different where mostly the available space is barely big enough for a body, and nothing much can be seen. 

For Ms McKinnon cave diving is a challenge, led by mental fortitude and physical endurance, backed by 12 years of cave diving experiences and more than 40 years as a caver and deep sea diver.

Only a handful of people are believed to be involved in Tasmanian cave diving.

Fellow cave diver Andreas Klocker, an oceangrapher who first began caving in Tasmania before training as a cave diver in the US, describes the state’s cave diving as “not pleasant”.

Ms McKinnon said generally “it tends not to be enjoyable”. 

“The sumps are less than a metre high, no bigger than a couple of body sizes big or smaller, and the water temperatures are about six to eight degrees,” Ms McKinnon said.

“You can’t swim through without stirring up the dirt and silt, so you lose all visibility, and you are in these small, tight passages where you may need to take your gear off to find a way through,” she said.

“Travelling through a passage with your tanks out in front of you, with zero visibility, is one fairly stressful situation.”

Cave divers must usually first work their way through dry caves, crawling or climbing over rocks and boulders, and abseiling down vertical pitches to perhaps 300 metres underground, to reach the desired, winding, water-filled sumps.

The aim is to dive through a sump to find new openings and unexplored dry caves, which in turn may lead to further sumps and further dry chambers.

All this is done while wearing an air tank, a mask, fins, a dry suit and various light sources.

Ms McKinnon said redundant equipment must always be carried, which means for every piece of equipment taken underground, an extra will be brought.

If the air in a tank runs out, or a problem occurs with the equipment, divers have a back-up.

“You always have two cylinders of gas … you never want to end up in a situation where you don’t have enough,” Ms McKinnon said.

While cave diving could be described as an extreme sport, many divers choose not to see it that way.

Deaths do occur, with one US paper documenting 161 cave diving deaths over 30 years, but they mostly occur when stringent diving protocols are incorrectly followed.

Ms McKinnon said training and preparation is critical to survival, including having a complete understanding of the possible risks involved.

She said the biggest risk was psychological, where panic must always be kept at bay to prevent the ultimate risk of drowning.

Physical risks include running out of oxygen, losing the guidelines and getting lost, becoming tangled in the guidelines, or dirt becoming trapped in the oxygen equipment.

“Panic kills more than anything else. If you are ten metres into a cave passage and an issue arises with your gear you need to be able to fix that problem,” Ms McKinnon said.

“You can’t just get back to the surface or to air the instant that you want.  Psychologically a lot of people can’t deal with that … You have as much gas as you have left in the tank to solve the problem or you die.”

Similarly, Mr Klocker said the main skill is to remain calm and avoid adrenaline.

He said losing the guideline, that he describes as “the holy string”, or becoming tangled up in it was a relatively easy thing to do.

“The guideline shows you the way back but obviously you can lose the string and you have to find it again to find your way out,” Mr Klocker said.

“One of the scariest things is not being able to see anything, you literally feel your way through the silt … and if you are tangled in the line, you have to cut yourself out without losing it.”

Ms McKinnon said divers must move slowly, carefully and methodically in the sumps. 

“It is about problem solving, thinking about how I’m going to be able to fit my body through...At no point do I want to suddenly feel as though I have pushed beyond what I can cope with, and if I ever start to think that it’s becoming tricky, or I’m not mentally happy, I quickly get out.”

Ms McKinnon said she enjoyed the long-term personal challenge that cave diving offered, where skills are gradually attained over time.

She said the pinnacle reward was pushing off into unexplored places where no one has ever been.

“There is a big difference between following a guideline someone else has put in, to being the first in and following your own line.  It is exciting and bit nerve-wracking.  Physically it is the same but mentally you are in a very different headspace,” Ms McKinnon said.

“The dive itself is dark, gloomy and cold but when you come up the other side, into a new chamber, and you are the first set of human eyes to have ever seen this, it is a pretty exhilarating experience.

“You sit there and think ‘wow, I am here, and it is just me in this place’.”

Mr Klocker said Tasmania was a great place to find unseen sumps and dry caves, with the state having some of the longest and deepest caves in Australia. 

“In Tasmania you have a lot of exploration potential, it is much easier to find a place where no-one has ever been.  In other countries such as the UK most caves are done and dusted, cave divers need to go overseas to find the new stuff, whereas I can just drive to the Florentine and be back home in a day.” 

The Junee-Florentine karst system in the south of Tasmania is known to cave divers across Australia, with its complex mix of caves containing water-filled passages, and a “master cave”.

Mr Klocker said divers must do a 230 metre sump dive of about 20 metres depth, before reaching the dry cave “For Your Eyes Only”.

“It is absolutely stunning, with stalactites and calcite straws coming off the ceilings in very beautiful formations,” he said. 

A second sump can be accessed from this dry cave, but cave dive explorers are yet to find a way through the many passageways and conduits to find new dry cave, or the elusive “master cave”.  

While divers have attempted for decades to find this cave an underground rock fall has continued to prevent access, thereby creating the ultimate challenge, and goading cave divers to continue their pursuits for the unseen. 

“You often have to turn around because you don’t find the way through, or it is too small, or visibility prevents you from getting through, but you often try multiple times,” Mr Klocker said.

When the moment of finding these new, underground, unexplored places does occur, it is defined by a mix of emotions.

A challenge solved but celebration withheld by the solitary nature of the pursuit, and the work that still lies ahead to resurface safely.

“Behind a sump, alone in a dry cave, that is when it feels really remote,” Mr Klocker said.

“You know you have to get back out but there is no one to help you.  You are on your own, and that is one of the most scary parts, knowing that you are cut off from help.  That is when you must really depend on your own skills.”