By Rebecca Wright, Kocha Olarn, Angie Puranasamriddhi, June Jeong, Laura Sharman, CNN
Vientiane, Laos (CNN) — Rescuers in Laos say they are investigating a possible “knocking response” deep inside a flooded cave network where two men are believed to remain trapped days after one was rescued and four others crawled to freedom.
The development could indicate possible signs of life but rescuers stress they need to investigate further before saying for sure whether the knocks are from the stranded men.
The response sounds came as rescuers rappelled down a newly discovered vertical shaft that may provide a safer entry into a flooded cave and lead to a chamber where the men could be.
“We still cannot confirm that the knocking sounds came from trapped victims. However, we can confirm that there were definite knocking sounds in response to the signals,” Thai specialist cave diver Kengkard Bongkawong told CNN by phone on Monday.
Rescuers have now heard knocking responses on at least two occasions in the last 24 hours.
Thai rescue technician Manat Artmongkron said that the response was heard 70 meters (230 feet) down the shaft.
“The sounds appeared to come from just 5 meters away, raising hopes that the missing individuals may be nearby,” he wrote Monday on Facebook.
The villagers’ ordeal began on May 20 when eight men entered a cave in central Laos in search of gold, but became trapped when heavy rain blocked the exit.
A survivor managed to raise the alarm, triggering a complex operation involving experts who were summoned from around the world to the country’s central Xaisomboun province.
Five men emerged alive from the tunnel on Friday and Saturday, leaving rescuers searching for the remaining two, who entered separately via a different route.
What’s next in the Laos cave rescue?
Attention turned Monday to the vertical cave shaft that plunges more than 100 meters into a dark chamber that rescuers hope will provide a safer entry into a flooded cave.
“It was an amazing find,” Australian diver Josh Richards said in a video clip late Sunday. “It’s also directly in the area we need to be connecting to.”
Rescuers are working with a map drawn with the aid of the five survivors and they hope the shaft will connect to another passage beyond the chamber where they were found.
“From what we know, there is a significant air pocket that is considerably further on – another 100 meters or so, through fairly lethal passage,” Richards said.
He described this stretch as an “even tighter and more unpleasant” passage than anything the divers have traversed so far, and the “only place” where the missing two could be.
Conditions at the cave entrance remain challenging, with the latest rainfall causing flash flooding.
Bounkham Luanglath, president of the Lao People’s Volunteer Association, said Monday that work continues to “pump water out to drain the cave as quickly as possible.”
Other methods include blocking water sources in the area and building more water retention ponds, on top of the existing one, to prevent further flooding.
Of the survivors, one was guided to safety on Friday, while the other four managed to escape unaided the next day after extensive efforts to drain the floodwater from the area.
All five are recovering in hospital after surviving for more than a week on water and sleep to preserve their strength before help arrived.
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