Some stretches of the Tham Luang cave are more than 10m (33ft) high, while others are a tight squeeze through water-filled passages.
The death of an experienced former Thai navy diver, who was part of a team trying to supply the boys' cave with oxygen tanks, emphasised the dangers of the mission.
The group of boys and their coach were exploring the caves when a sudden storm caused the passageways to flood, trapping them inside. They had spent nine days in the cave with little food or light when they were discovered on Monday 2 July.
Image copyrightAFP/ROYAL THAI NAVY
Rescuers initially wanted to keep the group supplied underground until the end of the rainy season - which would have taken months.
But with the forecast of heavy rains, and the risk of water levels rising, an operation was launched on Sunday, 8 July, to bring them out.
How did the boys escape?
Rescue divers with specialist breathing equipment reached the group through a series of water-filled passages.
The boys had to be brought out the same way.
The Thai government released a graphic showing how they planned to rescue the boys.