Rescuers in Thailand are trying to find alternative ways to rescue 12 boys and their soccer coach trapped inside a cave complex for 13 days, as teams brace for heavy rain which could further delay the operation.

“We are trying to find alternative techniques to reach the 13,” Chiang Rai Gov. Narongsak Osottanakorn told reporters late on Thursday.

“We have considered many alternatives to find the most feasible ones,” he added.

Rescue alternatives include teaching the boys to dive and then swim out, a highly risky venture, remaining in the cave for months until the wet season ends and flood waters recede, or drilling a shaft into the cave from the forest above.

The boys, aged between 11 and 16, and their assistant coach were found inside the Tham Luang cave in northern Chiang Rai province on Monday, after nine days underground, hungry but in good spirits. They went missing after they set out to explore the cave on June 23.

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Rescuers are now deciding how to remove the group but have been slowed down by logistical issues, including high water levels inside the cave and narrow, flooded passages that would require the boys to dive alone.

The Thai navy is now teaching the boys the basics of diving, with a view to guiding them out through flood waters.

But getting them out won’t be easy. The boys will have to be taught how to use scuba diving gear and how to navigate a cave that has frustrated even the most expert divers.

“Regarding the plan for the 13 to swim or dive, there is only one critical point which it is risky: It is where every boy has to dive alone. The point is very narrow. … It is very deep water. The distance is pretty long,” said Narongsak.

Some of the boys cannot swim.

But rescuers are also considering other options, including keeping the 13 inside the Tham Luang cave until the flood waters recede, at the end of the rainy season in about four months.

Others say the boys could be out in days if the weather is on their side and enough water can be pumped out of the cave to enable the boys to get out the same way they got in, on foot, perhaps with some swimming.

Another possibility would be to find an alternative way into their chamber, such as drilling a shaft into the cave from the forested mountain above.

Narongsak said rescuers were preparing a 5 km (3 mile) “oxygen pipeline” as part of preparations for the group’s extraction but added that the boys would not be coming out soon.

“You see we are increasing the number of people going inside the cave. So we have to fill it up with oxygen,” he said.

Heavy monsoon rains are forecast for next week in most of the north, according to Thailand’s meteorological department.