Dipping oxygen levels inside the cave complex have also raised alarm in the rescue operation.
More than 100 chimneys are being drilled into the mountainside in a frantic bid to reach a Thai youth football team trapped in a cave complex below, the head of the rescue mission said Saturday.
The unprecedented rescue effort is attempting to establish new ways to extract the boys from above, if the underground chambers flood and it is deemed too risky to evacuate the team by diving out through the submerged passageways.
"Some (of the chimneys) are as deep as 400 metres... but they still cannot find their location yet," Narongsak Osottanakorn told reporters, adding the mission lacked the technology "to pinpoint where they are staying".
"We estimate that (they) are 600 metres down, but we don't know the (exact) target," he said.
On the question of dipping oxygen levels in the cave, he said rescuers had managed to establish a line to pump in fresh air and had also withdrawn non-essential workers from chamber three -- where the rescue base is -- to preserve levels inside the cave.
The "Wild Boar" team have been trapped inside the Tham Luang cave complex for two weeks.
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