Tham Luang caves, Thailand (CNN) The perilous mission to evacuate 12 boys and their 25-year-old coach trapped deep inside a flooded cave in northern Thailand is under way, Thai authorities announced Sunday.

At 10 a.m. local time, an international contingent of 13 specialist divers and five Thai Navy SEALs descended into the watery network of underground tunnels below the Mae Sai mountains, carrying with them the hopes of an entire nation.

"Our readiness is at the highest today. Today is D-day," said Chiang Rai's governor, Narongsak Osotthanakorn, adding that the families had been informed of the plan and given their support to the rescue effort.

The plight of the stricken boys, who have remained trapped inside the caves for 15 days, has transfixed Thailand, as rescue efforts have become ever more urgent.

Their families sat glued to television sets and phones for updates Sunday, expressing hope that their boys would soon return home.

One of the boys, Peerapat Sompeangjai, turned 16 on the day the group went missing in the Tham Luang Nang Non cave complex, in Chiang Rai, northern Thailand.

On Sunday, his 17-year-old sister Phanphatsa told CNN she promised to make her brother another birthday cake -- and all his favorite food -- on his safe return.

Rescuers have a dwindling window of opportunity in which to extract the boys, with heavy rain falling shortly after the rescue operation began on Sunday afternoon and forecasters predicting yet more monsoon rains in the coming days, effectively sealing off the cave until October.

"We have two obstacles: water and time. This what we have been racing against since day one. We have to do all we can, even though it is hard to fight the force of nature," said Osotthanakorn, as a light rain began to fall across the site.

"All we need is a suitable time window when all conditions are right to carry out the operation, we have been waiting for this right moment."

Race against time

At the entrance to the caves, volunteers assisting in the operation described the rescue attempt as a "now or never" scenario.

The boys and their coach are huddled together in a small chamber four kilometers (2.5 miles) inside the cave, surrounded by flood water and with a limited supply of oxygen.

Osotthanakorn described the boys' physical and mental condition as good. "I can assure that they are ready and they are determined and ready to be extracted."

To reach them, divers will need to successfully navigate a network of sprawling, narrow tunnels connecting the boys' chamber with the rescue command center, known as Chamber Three.

Divers are expected to escort the boys through the tunnels one by one. Once they reach the command center, the boys will be handed over to separate specialist rescue teams, who will assist them through Chambers One and Two, according to a Thai navy official with knowledge of the rescue operation.

"Today the water level in chambers number One, Two and Three inside the cave is low enough to walk through them," said Osotthanakorn.

Rescue teams have been constantly pumping water out of the cave system, in a race against time before the return of heavy rain forecast for Sunday.

"The water of some parts recedes as much as 30 cm (11.8 inches), it is considered the lowest level for the past 10 days."

Signs that a rescue operation was under way were evident in the hours leading up to the announcement, as authorities installed a large green tarp around the cave entrance and removed media from outside of the camp to a separate location.

On the dirt track leading to the tunnels, a near continuous convoy of trucks and military vehicles delivered troops and medical equipment, including a large cache of oxygen tanks. On Saturday evening, numerous international military advisers could be seen entering the site, followed later by four monks in orange robes.

At the entrance to the site, a newly placed, thin white decorative flag fluttered in the wind, a Buddhist sign to indicate positive energy.

Photos: Photos: Thai soccer team rescued from cave The rescued team attends a news conference in Chiang Rai, Thailand, after being discharged from the hospital on Wednesday, July 18. Hide Caption 1 of 54 Photos: Photos: Thai soccer team rescued from cave Members of the soccer team greet well-wishers as they arrive for the news conference on July 18. Hide Caption 2 of 54 Photos: Photos: Thai soccer team rescued from cave The team poses with a sketched portrait of Saman Kunan, a former Thai Navy SEAL who died on July 6 while returning from an operation to deliver oxygen tanks to the cave. He ran out of air while underwater, an official said. Hide Caption 3 of 54 Photos: Photos: Thai soccer team rescued from cave Thai Navy SEALs pose for a photo after the rescue effort was finished on July 10. One of them was a doctor who stayed with the team for a week after the group was found alive on July 2. Hide Caption 4 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers carry one of the boys out of the cave. Hide Caption 5 of 54 Photos: Photos: Thai soccer team rescued from cave Divers wave through floodwaters in the cave. Each rescue took multiple hours. Hide Caption 6 of 54 Photos: Photos: Thai soccer team rescued from cave Chiang Rai Governor Narongsak Osotthanakorn speaks during a news conference held after the rescue was finished on July 10. Hide Caption 7 of 54 Photos: Photos: Thai soccer team rescued from cave A helicopter transports rescued boys to the hospital on July 10. Hide Caption 8 of 54 Photos: Photos: Thai soccer team rescued from cave People watch and cheer as a helicopter flies toward an airstrip to transport one of the rescued boys to a hospital. Four boys were rescued on July 8, another four were rescued on July 9, and the rest were rescued on July 10. Hide Caption 9 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers leave after all members of the team had been saved. Hide Caption 10 of 54 Photos: Photos: Thai soccer team rescued from cave This photo tweeted by tech entrepreneur Elon Musk shows rescue efforts during the three-day mission. Musk tweeted early July 10 that he had visited the cave and left a mini-submarine there for future use. Hide Caption 11 of 54 Photos: Photos: Thai soccer team rescued from cave Members of the Royal Thai Navy are pictured with the team inside the cave. Hide Caption 12 of 54 Photos: Photos: Thai soccer team rescued from cave The boys received medical attention after being found alive on July 2. Hide Caption 13 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance believed to be carrying one of the rescued boys heads to the hospital on July 10. Hide Caption 14 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers walk toward the entrance of the cave as rescue operations resumed on July 10. Hide Caption 15 of 54 Photos: Photos: Thai soccer team rescued from cave Hospital staff and a police officer are seen during a news conference that was held on July 10. The eight boys that had been rescued on July 8 and 9 were being treated in an isolation ward in a Chiang Rai hospital. Hide Caption 16 of 54 Photos: Photos: Thai soccer team rescued from cave Thai police and military officers use umbrellas to shield a rescued boy before he was transported to a hospital on July 9. Hide Caption 17 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance exits the cave area on July 9. Hide Caption 18 of 54 Photos: Photos: Thai soccer team rescued from cave Emergency responders surround a helicopter on July 8. Hide Caption 19 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue teams arrange a water-pumping system at the cave's entrance on July 7. Hide Caption 20 of 54 Photos: Photos: Thai soccer team rescued from cave Ambulances wait outside a hospital where the rescued boys were treated. Hide Caption 21 of 54 Photos: Photos: Thai soccer team rescued from cave Thai military personnel walk into a cave during rescue operations. Hide Caption 22 of 54 Photos: Photos: Thai soccer team rescued from cave Military personnel work inside a cave. Hide Caption 23 of 54 Photos: Photos: Thai soccer team rescued from cave An ambulance leaves the scene of the rescue effort on July 8. Hide Caption 24 of 54 Photos: Photos: Thai soccer team rescued from cave This undated photo, released via the Thailand Navy SEAL Facebook page, shows rescuers with their hands locked. The caption said, "We Thai and the international teams join forces to bring the young Wild Boars home." The Wild Boars is the name of the soccer team the boys play on. Hide Caption 25 of 54 Photos: Photos: Thai soccer team rescued from cave Family members pray in front of a Buddhist statue near the cave on July 8. Hide Caption 26 of 54 Photos: Photos: Thai soccer team rescued from cave Emergency workers carry oxygen tanks as rescue operations begin. Hide Caption 27 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers work to connect pipes that help water from entering a cave. Hide Caption 28 of 54 Photos: Photos: Thai soccer team rescued from cave Thai forest rangers examine a map as they review their rescue options on July 8. Hide Caption 29 of 54 Photos: Photos: Thai soccer team rescued from cave This image, taken from video on July 2, shows the team trapped inside the cave. That was the day divers found them alive. Hide Caption 30 of 54 Photos: Photos: Thai soccer team rescued from cave Divers were able to bring food, blankets and other supplies to the boys while experts determined the best way to get them out safely. The team was found around two kilometers (1.24 miles) into the cave and somewhere between 800 meters to one kilometer below the surface, according to a British Cave Rescue Council briefing note. Hide Caption 31 of 54 Photos: Photos: Thai soccer team rescued from cave The team was found alive by British divers. The search brought rescue teams from all over the world. Hide Caption 32 of 54 Photos: Photos: Thai soccer team rescued from cave A Thai military medic provides first aid to one of the boys. Hide Caption 33 of 54 Photos: Photos: Thai soccer team rescued from cave The body of Saman Kunan, a former Thai Navy SEAL, is carried during a repatriation and religious rites ceremony on July 6. Kunan died Friday as he returned from an operation to deliver oxygen tanks to the cave. He ran out of air while underwater, an official said. Hide Caption 34 of 54 Photos: Photos: Thai soccer team rescued from cave Thai police take a break for breakfast as rescue operations continue on July 4. Hide Caption 35 of 54 Photos: Photos: Thai soccer team rescued from cave Water is pumped out of the cave on July 3. Hide Caption 36 of 54 Photos: Photos: Thai soccer team rescued from cave Workers fix the road leading to the cave on July 3. Hide Caption 37 of 54 Photos: Photos: Thai soccer team rescued from cave Divers prepare a light to be used for the underwater search on July 2. Hide Caption 38 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers wait at the entrance to the cave on July 2. Hide Caption 39 of 54 Photos: Photos: Thai soccer team rescued from cave Some of the team's family members celebrate after receiving news that the team was alive. Hide Caption 40 of 54 Photos: Photos: Thai soccer team rescued from cave Technicians lift water pumps to the drilling site on July 1. Hide Caption 41 of 54 Photos: Photos: Thai soccer team rescued from cave Soldiers and rescuers work outside the cave complex on July 1. Hide Caption 42 of 54 Photos: Photos: Thai soccer team rescued from cave Monks attend a Buddhist prayer for the team on July 1. Hide Caption 43 of 54 Photos: Photos: Thai soccer team rescued from cave Scuba tanks are delivered to the search site on July 1. Hide Caption 44 of 54 Photos: Photos: Thai soccer team rescued from cave Rescue workers carry water-pumping equipment into the cave on July 1. Hide Caption 45 of 54 Photos: Photos: Thai soccer team rescued from cave A Thai Air Force worker drops near a possible cave opening on June 30. Hide Caption 46 of 54 Photos: Photos: Thai soccer team rescued from cave British cave diver Robert Charles Harper explores an opening on June 29. Hide Caption 47 of 54 Photos: Photos: Thai soccer team rescued from cave Relatives of the trapped team members pray on June 27. Hide Caption 48 of 54 Photos: Photos: Thai soccer team rescued from cave Family members waited more than a week for news of their loved ones. Hide Caption 49 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers bring hoses and additional water pumps as the search for the team continued on June 27. Hide Caption 50 of 54 Photos: Photos: Thai soccer team rescued from cave Volunteer paramedics wait near the cave entrance on June 27. Hide Caption 51 of 54 Photos: Photos: Thai soccer team rescued from cave A rescuer looks for a cave entrance on June 27. Hide Caption 52 of 54 Photos: Photos: Thai soccer team rescued from cave Rescuers search the cave on June 26. Hide Caption 53 of 54 Photos: Photos: Thai soccer team rescued from cave Thai soldiers relay cable at the search site on June 26. Hide Caption 54 of 54

High risk

Hopes had been high that an alternative means of rescue would be discovered. For days, specialist teams have scoured mountains above the cave in search of a possible hidden entry point.

Divers have previously described conditions in the cave network as some of the most extreme they have ever faced.

The decision to remove the boys using divers would not have been taken lightly. On Friday, a former Thai Navy SEAL died while returning from an operation to deliver oxygen tanks to the cave where the boys are located.

Even if the divers are successful, it will be many hours before the fate of the boys and their rescuers will be known, with Thai authorities suggesting the extraction process could take days.

"We estimate the extraction of the first boy will be finished by 9 p.m. local (10 a.m. ET)," Osotthanakorn said.

It has previously taken divers 11 hours to complete the round trip.

Men from the Department of Disaster Prevention and Mitigation arrive to the cave entrance on July 6, in Chiang Rai, Thailand.

Finnish volunteer diver Mikko Paasi, a long-term resident of Thailand, said the death of the Thai Navy SEAL had changed the mood on the ground and made real for rescuers just how dangerous the mission had become.

"Definitely, you can feel it that it has an effect, but we're moving on. Everyone is a professional so we're trying to put it away and avoid it happening again," he said, adding: "Everybody is focusing on getting these boys out -- keeping them alive or getting them out."

At the Chiangrai Prachanukroh Hospital, where officials plan to send the boys after they are extracted from the cave, a number of gurneys have been placed outside the front of the main entrance in anticipation of their arrival. The hospital, the largest in the region, is approximately 57 km (35.4 miles) from the cave.

.@CNN has witnessed gurneys being prepared at a local hospital for the eventual arrivals of the soccer team, once each of the 13 members are rescued from the cave #ThaiCaveRescue pic.twitter.com/9yOaWRCwJ4 — Matt Rivers (@MattRiversCNN) July 8, 2018

Spirits high

In the hours preceding the rescue, a letter the boys had sent to their families was published on the Thai SEALs' Facebook page. The letter shows the boys in good spirits despite their ordeal.

In neat blue handwriting, 11-year-old Chanin Viboonrungruang, the youngest of the group, told his parents not to worry, and said he was looking forward to eating fried chicken.

His parents, who along with other families, have maintained a constant vigil at the site since the boys first became trapped.

Chanin Viboonrungruang, 11, is in his final year of Anubanmaesai primary school in Mae Sai.

On reading the letter Saturday evening, Chanin's father, Tanawut Viboonrungruang, said he felt a great sense of relief. "I had been worried about my son, that he would be exhausted, he would be tired," he said.

For families the wait for news of their boys' rescue has been excruciating.

"I was worried about him as there are obstacles to extract him out. Everyone knows that it is difficult to stay inside (the cave) but the officials are trying (to help him)," said Viboonrungruang.

"I hope he is still healthy and would come out soon. I want to send my support to him. I don't have a chance to talk with him," he said.