Right now, an underwater tunnel is the only way out for 12 boys and their soccer coach who have been trapped deep in a Thai cave for nearly two weeks.

Falling oxygen levels in the cave have made rescue more urgent. Teaching the boys enough diving skills to swim out with an experienced rescuer is one of the options that has been proposed.

But it is the "most dangerous option," according to Anmar Mirza, national co-ordinator of the U.S. National Cave Rescue Commission in an opinion piece on CNN earlier this week.

Drowning in the arms of the rescuer is a very real risk. — Jill Heinerth, Royal Canadian Geographical Society

That danger was underlined by the death of a former Thai Navy SEAL while diving inside the cave Friday. That evening, Thai Acting Governor Narongsak Osottanakor told reporters the boys are not yet ready to make the dive out.

The boys, ages 11 to 16, and their 25-year-old coach have been trapped about a kilometre underground, deep in Tham Luang Nang Non cave in Chiang Rai province since June 23. The cave was flooded by monsoon rains as they were exploring it. The group was finally found alive 10 days later but, since then, local and international teams of rescuers have been trying to figure out how to get the boys out.

Some proposals include:

Having experienced divers bring them food and other supplies while pumping water out of the cave until the rainy season ends in a few months and floodwaters subside — something that worsening weather and dropping oxygen levels are making riskier.

in a few months and floodwaters subside — something that worsening weather and dropping oxygen levels are making riskier. Trying to dig a tunnel from above, once their exact location is known.

Finding another exit, which hasn't worked out so far.

Many other people around the world, including Tesla and SpaceX founder Elon Musk, have tossed or tweeted other ideas on social media.

'Extremely risky'

"The public seems to be weighing in favour of diving out of the cave," wrote Jill Heinerth, an experienced cave diver and explorer-in-residence for the Royal Canadian Geographical Society in a blog post Thursday.

"But as a cave explorer myself, I have to caution that this is extremely risky."

It would be considerably more difficult than walking or crawling through a dry cave with a light that allowed them to see around them, as the boys did on the way in.

"For divers wearing equipment… we are bigger, there is more gear to get entangled than a nearly naked kid," Heinerth said in an email to CBC News on Friday.

Diving also takes a lot of physical exertion, due to the drag of the water and the extra resistance from the equipment, not to mention the current in some cases. That could be hard on the boys, who are weak from barely moving or eating since becoming trapped.

A man dives between rock formations at Isola Bella in Taormina, Italy, in July 2015. Diving takes a lot of physical exertion, because of the drag of the water and the extra resistance from the equipment. (John Schults/Reuters)

The water in the tunnels is also very muddy, providing very little visibility. The current is strong — the cave has basically become an underground river, rushing downhill. And rescuers have reported very tight constrictions that need to be squeezed through as they navigate the twisting tunnels of black water.

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Any rescuers would have to carry each boy through the tunnels in those conditions, while feeling for a thin, nylon guideline for navigation.

"Drowning in the arms of the rescuer is a very real risk," Heinerth wrote. "The best cave divers in the world can make the exit in four hours, but with a child in their arms, it will take considerably longer."

Cave precautions

Matt Mandziuk, a diving instructor at Dan's Dive Shop in St. Catharines, Ont., has been cave diving for 18 years. He gives courses on it, and says divers need very good open-water skills before trying to dive inside a cave.

What makes cave diving so dangerous is there is no option to surface if something goes wrong. Because of that, cave divers carry much more equipment for redundancy, he said — such as two independent regulators on two independent tanks, three lights instead of one, cutting tools and a couple spools of guidelines, which are similar in strength to fishing line and can get cut rather easily.

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Using blacked out masks, they learn to communicate with poor or no visibility — using light signals, hand signals and pushes and pulls.

A diver checks his mask on a rescue ship in the Java Sea in January 2015. A former Thai navy SEAL died while diving inside the cave Friday, underscoring the danger of rescue efforts. (Beawiharta/Reuters)

They practise changing masks underwater and sharing compressed air in case a teammate runs out — all while maintaining perfect buoyancy so they don't hit their heads on the ceiling or kick up too much debris from the bottom and further spoil the visibility.

"It's a very intense course," Mandziuk said.

The idea is to create muscle memory and desensitize students against the cave environment so they can take the correct actions calmly, he said. Panicking can use up air very quickly.

In the case of the cave in Thailand, navigating the boys through the narrowest parts without anyone getting stuck or consuming too much air, especially if they need to take their gear off to fit, will be a "challenge," said Mandziuk.

"But I still think if they do it properly, it's still feasible," he said.