Tethered to a frogman, with a static guide rope and stage tanks along the passage, the 12 trapped Thai schoolboys will attempt to escape the cave where they are trapped in a perilous rescue mission.

The treacherous four-hour journey to safety will take at least two days of continuous individual rescues. And the dangerous task could be further complicated by health issues, with two boys and their coach reportedly suffering from exhaustion due to malnutrition.

To prepare for their journey out of the cave system, each 11- to 16-year-old will first be fitted with a wetsuit, aqua boots, a helmet and a scuba mask.

The boys smile as Thai Navy SEAL medic help injured children inside a cave in Mae Sai. Photo / AP
The boys smile as Thai Navy SEAL medic help injured children inside a cave in Mae Sai. Photo / AP

Each boy will be briefed on what is about to take place to help minimise the risk of panic, and escorted by two Thai Navy SEALs as they make their risky exit.

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Instead of having their own scuba tanks, the boys will have to rely on an individual navy rescue diver's air supply shared with them along the way.

The divers will use a static rope attached to the cave walls to help guide each boy one-by-one through the cave system of completely flooded chambers and those with air pockets.

Oxygen "stage tanks" have been placed every 25 to 50 metres along the linked system of cave chambers so that the boys can get extra oxygen if needed.

The flooded passage passing through the linked caves is filled with hazardous bends and next to no visibility, with water in some parts being described as "black coffee".

The passage is the biggest challenge facing the trapped schoolboys as authorities race to drain water from the flooded Tham Luang Nang Non cave in northern Thailand's Chiang Rai province.

As part of a desperate attempt to save the boys, Thai authorities are also trying to confirm whether there is a secret passage to safety.

The world was watching with bated breath as it was hoped an attempt to free the 12 starved boys and their 25-year-old football coach would be made on Thursday. However, the rescue operation has been postponed at least another day.

"We hope that today or tomorrow the water level between the third chamber and the point that the students are at will decrease more," a senior Thai rescue official said, according to Fairfax.

"The water level between the third chamber and the boys is still high. We put the tubes at the third chamber, trying to get the water out as much as possible."

The boys are reportedly being trained in how to breathe with scuba equipment.

But diving experts have warned that the mission is extremely dangerous as the boys can't swim and have never used scuba gear, and the cave's narrow passageways pose a significant challenge for even the best cave divers.

A narrow 3km flooded stretch of the Tham Luang Nang Non cave, which took experienced cave divers about three hours to navigate, poses an ominous hurdle for the inexperienced boys in the desperate rescue plan.

British Cave Rescue Council chairman Gary Mitchell told the BBC that freezing, muddy water was touching the roof of the cave inside the pitch black passage, which is extremely narrow — so much so that it is only big enough to fit one person through at a time.

A Thai diving website, Digitalay, has posted a sketch that shows how divers must tackle the dangerous 3km stretch.

Rescuers lay telephone cable from a cave where a young football team and their coach are trapped. Photo / AP
Rescuers lay telephone cable from a cave where a young football team and their coach are trapped. Photo / AP

Volunteers holding a rope guideline would be submerged by 10m at each flooded point in the passage and divers would have to dismount their scuba kits to stand any chance of getting through.

As each boy is dragged through the dangerous passage while closely tethered to a diver navigating the tight nooks and crannies, there is a risk of their gear being knocked off.

The diving site reported that Belgian cave diver Ben Reymenants, who owns a business in Phuket, had discovered and placed a guideline in a new, wider channel with occasional air pockets, which should make the extraction easier and safer.