A rope was eventually strung through the flooded sections to provide a guideline for the divers and, in the drier sections, a physical connection between the surface and the boys. The Thai Navy SEALs who had entered the cave on July 2 and remained with the team were receiving regular resupply of their own gear, food, and equipment through the same route.
The communication established during this period was more than logistical. The families of the boys, who had been waiting outside the cave since June 23, were given the opportunity to write letters to their children. Those letters went in with the supply runs. The boys wrote back. Seeing evidence of their children’s handwriting — cramped from two weeks of cold and darkness, but unmistakably alive — was a form of information that the families had been waiting for since the moment the cave flooded.
The diving teams conducting the supply runs were accumulating something else with every pass through the passages: knowledge. They were learning exactly how the water levels responded to rainfall, how the visibility changed over the course of a day, where the current ran strongest, where the guideline caught on the cave walls. When the extraction plan was formalized, the divers executing it had made dozens of transits through the passages they would use. The cave that had been alien and unknown on July 2 was, by July 8, as known as it could be made.
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## Oxygen and the Air That Almost Ran Out
One of the least-discussed dimensions of the survival story is the oxygen situation inside Nern Nom Sao. It emerged in reporting after the rescue that the oxygen level in the chamber had dropped significantly during the team’s time there — to approximately 15 percent at some points, compared to the normal atmospheric level of 21 percent.