A lot of unknowns continue to surround the precarious situation in northern Thailand, where a group of boys remain trapped deep underground in a cave system with floodwaters rising. As rescuers place the mission on hold to restock supplies, the team will soon have a new option at their disposal in the form of a kid-sized submarine built by Elon Musk and a team of SpaceX engineers.

The effort to rescue the 12 members of a youth soccer team and their coach has been called a "war against water and a race against time." As monsoon rains set in this week they bring a very real risk of water levels rising inside the cave system, further complicating what is already a very challenging exit strategy.

Expert cave divers have managed to rescue four of the boys so far, a trip out that took eight hours and involved climbing, wading and diving through confined sections, the narrowest of which measures just 38 cm (15 in) wide. Slipping through that tiny gap meant removing the scuba tanks and pushing them and the boys through one at a time.

While four members of the soccer team have now been freed, there are no guarantees with how the remainder of the mission will play out. Rescuers are now replenishing oxygen tanks and gearing up to go again, and they may or may not choose to make use of a miniature submarine built by the SpaceX engineers in LA after liaising with the dive team in Thailand.

The robust submarine is built to accommodate a kid or small adult and features the liquid oxygen transfer tube of the Falcon 9 rocket as its hull. It weighs 40 kg (88 lb) when dry, so is light enough for divers to carry across dry sections of the caves. It has a diameter of 31 cm (12.2 in) and, according to Musk, is built to be pushed and pulled through the tightest parts of the system.

"According to divers who have made the passage, yes," Musk tweeted when asked whether it could make it through the cave's so-called "choke point." "However, we also made an exact replica that is inflatable, so that the entire path can be tested without risk of blockage."

The submarine was constructed over around eight hours and has been put through testing in an LA pool with a person inside, including maneuvering through obstacles that replicate the cave's narrowest passages. It is now being flown to northern Thailand.

"Mini-sub arriving in about 17 hours," Musk tweeted. "Hopefully useful. If not, perhaps it will be in a future situation."

View gallery - 3 images