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. FOLLOW LIVE UPDATES 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. 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. 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. 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. 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.