Parents always tell their children to stay away from the notorious Tham Luang cave network in the north of Thailand. Right outside the entrance are signs warning visitors to not take the risk.

Locals know that during monsoon season, the cave system can rapidly flood with water — which is exactly what happened on June 23 when 12 boys and their soccer coach became stranded in the labyrinth.

The international rescue attempt took a tragic turn earlier today when a former Thai Navy SEAL diver, who had volunteered to help with the efforts, died from a lack of oxygen.

And as time continues to tick down for the stranded group, their anxiously waiting friends have begun to worry why the boys decided to risk their lives in the first place.

Speaking to CNN at the local school in the north of Thailand, one of the friends said everyone knew not to go into the infamous cave system, especially during monsoon season.

"I was very worried about what would happen to them. The caves are a dark and scary place. I wouldn't dare to ever go in there," Kittichoke Konkaew, 14, told CNN.

Kittichoke is waiting and hoping his close friend, Nuttawut Takumsong, will emerge from the cave safely.

The group is more than 3km inside the cave system and was reached by expert divers earlier this week.

But two weeks since they first wandered into the labyrinth, the boys and their coach are still stranded — stuck in the same opening by seasonal flood waters.

The cave system's notoriety has seen it become a magnet for teen adventurers. According to locals, boys regularly explore the cave system for entertainment.

A Dutch diver assisting with the rescue said the 12 boys were led into the caves by their soccer coach as part of an initiation ritual where they inscribe their names on one of the rock faces.

Even if they hadn't becoming stranded, the boys were in for a precarious journey when they ventured inside the cave system.

The rocks are slippery and wet and there are numerous ways to travel throughout the system. There are cliffs with stark drop-offs and all of the exploring is done in pitch black.

Ever since the group of 13 went missing on June 23, students have spent their mornings praying for the friends before heading into classes.

Both teachers and students at the local schools have been anxiously waiting for their friends to emerge from the system.

"My students were sad. They cried when they heard about the news," said Worawit Chaiga, who teaches Nuttawut, another of the stranded boys. "It was all anyone talked about."

The young teacher told CNN he went back to the cave every night to ask for information about the missing boys.

"I couldn't believe this would happen to my students. In the classroom, I brought this subject up to teach students. You see, look at this event … it happened because they were not being careful, their actions has caused a lot of worry and create all kinds of issues to all," he said.

Another teacher, Wimonchat Jittalom, spoke about how proud he was of his student Ardoon Sam-aon, another of the trapped boys.

Mr Jittalom teaches Ardoon English, and said it was the 14-year-old who perfectly understood and answered a British diver when he asked how many people were in the opening.

"I was so proud," Mr Jittalom said. "Not only did he understand the question, but he was able to answer correctly!"

Rescue teams are attempting to get all 13 out safely as floodwaters continue to rise and oxygen levels dwindle.

Local media is reporting the diver who died earlier today is retired Navy Petty Officer 1st Class Saman Kunan who volunteered for the operation after having left his post.

The rescuer lost consciousness after placing oxygen tanks in the cave and could not be revived, despite first-aid attempts.

Authorities say the cave system is losing oxygen as they continue to try and get air supplies to the kids and coach through a pipe.

The boys have been given crash courses in specialist diving but experts say taking them out that way is among the most dangerous means of exiting the caves.

Even the expert Navy divers take up to five hours to swim through the jagged, narrow tunnels from where the boys are, to safety outside.

Despite the worrying road ahead, the boys' classmates are not concerned.

"Of course they can do it. They can swim. They will be fine," Puwadet Kumngoen said.