The first group of 12 Thai boys trapped in a flooded cave with their soccer coach was safely extracted Sunday as a risky rescue mission began ahead of heavy rains. Four boys were said to be above ground and undergoing medical examinations just a few hours after operations began, according to local officials cited by BBC. The four were identified as Nattawut Thakamsong, 11, Prajak Sutham, 14, Pipat Bodhi, 15, Peerapat Lompiangchai, 16. Thai authorities said the second phase of the rescue mission will begin in 10-20 hours. The healthiest boys were reportedly taken out first, with those remaining inside thought to be in worse shape. Authorities have said the evacuation could days. Following days of planning, Thai officials finally gave the go-ahead Sunday for a diving mission out of fear they might not have another chance. “If we keep on waiting and the rains come in the next three or four days, our readiness will decrease,” Chiang Rai acting Gov. Narongsak Osatanakorn said, adding that “today is D-Day.” Thirteen ambulances and helicopters are on standby outside the cave to to transport the boys and their coach to a hospital once they are rescued. The operation to save the group comes a day after the release of heart-wrenching letters the boys wrote to their families.