They came from all over the world, willing to risk their lives.
The contingent of rescuers includes a team of Thai Navy SEALS, a highly-trained corps of emergency operatives willing to trade their lives for those of the trapped teenagers.
“SEALS are used to going in and risking their lives for others’ lives,” Don Mann, a former U.S. Navy SEAL, told ABC News.
The acronym stands for "Sea, Air and Land," representing the different environments in which SEALS are trained to operate.
“They do that all the time," Mann said. "That’s why they sign up. Even when they’re training they are risking their lives -– that’s how rigorous the training is.”
The divers were all drawn to northern Thailand to join the massive rescue operation to extract the Wild Boar teammates and their coach from Tham Luang Nang Non, the country's longest cave. They had been trapped in the mountainous cave in Chiang Rai province since June 23.
The Thai cave rescue -- even for the most highly-trained divers -- is daunting.
“Even if you’re a Navy SEAL diver, or a civilian diver and you go through rescue training, it doesn’t even scratch the surface of what they are trying to accomplish during this rescue," said Mann, author of “How to Become a Navy Seal: Everything You Need to Know To Become a Member of the US Navy’s Elite Force.”
“These guys are highly-trained but these poor young kids -– what they have to do, to learn in a very short amount of time is to learn to swim and scuba dive," he said.
"And they’ll have two rescue divers along each side of each of the boys, but they’ll be swimming through tight, tight narrow spaces and black passageways that," he said, pausing to find the right word. "You get very claustrophobic.
"It’s a very terrible feeling,” he added.
The team rescued four of the boys Sunday, and were preparing for the second phase of the daring mission in several hours. Before the mission, some rescuers shared a photo on Facebook linking arms, with the message, "We, the Thai team and the international team, will bring the Wild Boars home," a reference to the soccer team's name.
But the recent death of a former member of the Royal Thai Navy inside the cave who was working as a volunteer rescuer was an emotional blow to the operation.
Saman Gunan lost consciousness underwater during an overnight operation delivering extra air tanks inside the cave, along the treacherous route divers take to get to the trapped soccer team. He could not be revived and was confirmed dead early Friday morning.
Gunan, 38, formerly served in the Royal Thai Navy’s Underwater Demolition Assault Unit, colloquially known as the Thai Navy SEALs. His death marked the first fatality in the operation to rescue the group and underscored the dangers of navigating through the cave underwater, even for those who have experience.
Gunan's body was sent back to the naval base in Sattahib district, where the king of Thailand has ordered a funeral to be held with full honors, according to Thai officials.
Rescuers like Gunan are generally brave, selfless people to begin with, Mann said.
“They are people who have character,"he said, "who go in [to a rescue operation] with the notion that what they’re going to do is risk their lives to get to save others’ lives.”