New Thai cave rescue details: 'We didn't think any would survive'

PONG PHA, Thailand -- New details are emerging about the miraculous rescue of 12 boys and their soccer coach from a flooded cave in Thailand.

According to 20/20, the U.S. Air Force major involved in the rescue operation told Thai officials he'd deem the operation a success if just one of the 12 boys made it out alive.

Here's more of what ABC News has learned about the impossible rescue:

• A rescue planner says that they "didn't think any would survive" the journey out of the cave.
• The Thai government had hoped to wait out the monsoon or drill a rescue shaft from above in hope of having a "zero risk" option.
• British rescue divers, Australian doctors and the USAF special ops planners said that if they waited for the monsoons to end or waited the six to eight weeks to drill a rescue shaft, the boys would die.
• The oxygen level had reportedly dipped to just over 15 percent in the cave at that time, which was very dangerous, and the cave was filling with carbon dioxide.
• Officials got the go ahead for the rescue on Saturday, July 7, and the mission started Sunday morning, July 8.
• The U.S. major in charge, Maj. Charlie Hodges, said that just one child coming out alive would be a success.
• The Thai Navy seal commander told ABC News that they didn't expect any of the children to survive and that 50 percent would have been good.
• British cave divers reportedly said that the boys had to be completely unconscious for the rescue or they wouldn't go ahead with it. There was one diver per boy. Ketamine was administered to sedate the boys, so they felt and saw nothing during the rescue.
• On the first dive, just a few hundred meters in, rescuer Jason Mallinson said the boy he was rescuing regained consciousness, so he had to pin him to the cave wall while he grabbed a syringe to sedate him.
• The last boy out was reportedly hypothermic. He would have been the second to last out, but his diver lost the guide line and became lost.

Matt Gutman reveals more incredible details during a special episode of 20/20 Friday on ABC.