July 9 (UPI) -- Four more boys were pulled from a deep cave in Thailand Monday, leaving four boys and their adult soccer coach underground in rising floodwaters.
The second phase of the rescue effort began on Monday, hours after the first four were pulled to safety late Sunday. One of the boys was removed by divers, placed on a stretcher and hospitalized. Divers removed another boy about an hour later. Two more were taken from the cave before nightfall.
Monday's mission began with eight boys and their 25-year-old coach still underground, all of whom have spent more than two weeks in the cave in Chiang Rai province.
It took hours to place oxygen tanks and guide ropes along the narrow tunnel leading to the boys. Additionally, a weir, or low dam, was built outside the entrance to keep water levels low, as heavy monsoon rains threatened to fill it and the 2.4-mile escape route.
The four boys removed Sunday were hospitalized in stable condition at Prachanukroh Hospital, about 35 miles from the cave. The hospital's eighth floor was reserved entirely for the soccer team, whose players range from ages 11 to 16. The four evacuated on Monday were also hospitalized, although their condition was not announced.
Families have maintained a vigil at the cave entrance since the group became trapped June 23. The families have not yet been able to interact with the boys out of fears of infection.
It took nine hours Sunday to rescue the first two boys, who were described as the weakest of the group. Thai Navy SEALs later announced two more had been freed in what Gov. Narongsak Osatanakorn called a "very smooth operation."
Thirteen divers and five Thai Navy SEALs are involved in the operation, in which the trapped team must walk or wade through muddy areas of the winding cave and rising waters.
Progress came more easily on Monday, with replenished oxygen tanks and tightened guide ropes. Ninety divers are involved in the rescue.
The group was stranded for nine days before two British divers discovered them in their present location. They became trapped when heavy rains produced flooding underground, a common occurrence at the cave this time of year. A Thai naval diver died of asphyxiation Friday while placing oxygen tanks in the cave. His death accentuated the hazards of the rescue and added a greater sense of urgency in getting the boys out.
Several countries, including the United States, have sent cave experts and divers to aid in the rescue. SpaceX founder Elon Musk said a 90-pound, "kid-sized submarine" is on its way from Los Los Angeles to Thailand for possible use.
Monday Musk shared a video of the craft, made from a part of SpaceX's Falcon 9 rocket, being tested in a swimming pool.
Simulating maneuvering through a narrow passage pic.twitter.com/2z01Ut3vxJ- Elon Musk (@elonmusk) July 9, 2018