MAE SAI (Thailand) • The authorities highlighted the tiny passageway near a T-junction as the most dangerous element of the route through which 12 schoolboys and their football coach must navigate to escape from a flooded cave.
Here is a look at the junction - Sam Yaek in Thai - and the other potential pitfalls of the perilous route to safety.
The boys, aged between 11 and 16, have no diving experience and some cannot even swim. They have received training in recent days in preparation for the extraction effort, but they will have to swim using scuba gear through fast-flowing water in darkness - a challenge for even elite divers.
The difficulty of the journey was underscored when a former Thai navy Seal diver died last Friday after running out of oxygen in the cave.
The boys' weakened state - they were found in an emaciated state nine days after they ventured into the cave, although they have been receiving food and medicine since then - also could be a crucial factor in determining their fate.
Thirteen foreign divers and five from Thailand's navy Seal unit are involved in the rescue effort. Two divers will escort each of the boys and the 25-year-old assistant coach.
The sliver of space is 1.9km from the shelf where the boys have been sheltering above the waters.
Number of foreign divers - in addition to five from Thailand's navy Seal unit - involved in yesterday's rescue.
After energy-sapping efforts navigating jagged tunnels and clambering up or down rock walls for this distance, they will confront Sam Yaek.
"The biggest crisis spot for diving is on the left from the T-junction," said Mr Narongsak Osottanakorn, the rescue mission chief, in a briefing on July 2.
"There is a tunnel that has a passageway going up and coming down narrowly and you have to turn a bit and it's very small."
After that though, the tunnels widen, the waters subside and walking is even possible, according to the authorities. The rest of the journey is expected to be relatively safe as they will have reached a forward operating base inside the cave.
The water in the cave is muddy and unclear, with one diver comparing it to a cafe latte. The labyrinth has no outside light. The boys will be helped through the darkness by guiding rope, torches and their escorts.
Nevertheless, the poor visibility is one of the factors raising concerns about the boys - already traumatised after spending so long in the cave and having to swim underwater - potentially panicking.
"The mental side of this has to be one of the top considerations," Mr Andrew Watson, an experienced rescuer of mine workers, previously told Agence France-Presse. "Just one individual panicking can cause a problem."
The operation was launched after several days of relatively mild weather, as more than 100 million litres of water were pumped out of the cave.
Mr Kobchai Boonyaorana, deputy director-general of the Disaster Prevention and Mitigation Department of the Interior Ministry, told reporters yesterday that the water level in the cave had continued to recede, and that rainfall was less than expected.
However, weather forecasters warned that heavy rain was on the way, which could flood the area completely.
They said there was a 60 per cent chance of moderate to heavy rain yesterday afternoon, adding that heavier rain would continue from today to Thursday.