Vientiane, Laos (CNN) — Rescuers in Laos are racing against time to find two men trapped in a flooded cave network for almost two weeks, with efforts turning to a search of dense jungle to find another way in.

The men are thought to have entered the cave on May 20, just before heavy rain caused flash flooding, blocking their exit. Five others escaped the cave’s narrow tunnels after rescue efforts by international experts summoned to the perilous cave in central Laos.

Hope was raised Monday after rescuers reported hearing a possible “knocking response” deep inside the cave network, ?and finding a vertical shaft they hoped would lead to a chamber where they’d find the men. However, as the hours wore on, both leads appeared to be dismissed.

Thai specialist cave diver Kengkard Bongkawong had told CNN by phone earlier that “there were definite knocking sounds in response to the signals” though it wasn’t confirmed the sounds came from trapped victims.

Thai rescue technician Manat Artmongkron said the response had been heard 70 meters (230 feet) down the vertical shaft.

Australian diver Josh Richards later dismissed the reports, telling CNN the noise “could have been bats” or “the wind coming in.”

“A lot of these caves have fissures through them as well, or very unusual sounds coming down from below. They didn’t sound like they were echoes or anything like that,” he said.

Searching for another entry

The earlier reports of response sounds came as rescuers rappelled down the newly discovered vertical shaft that they hoped could provide a safer entry into the flooded cave.

Early assessments suggested the shaft dropped more than 100 meters (328 feet) into the cave, potentially leading to a chamber where the men could have sought refuge.

At that stage, rescuers were working with a map drawn with the aid of the five survivors. A team of divers was waiting for authorities to bring advanced scanning equipment that can produce a clearer map of the cave, Kengkard told CNN.

Rescuers could potentially use that equipment while being lowered down the shaft, which would provide more detail about the cave’s layout, added Kengkard.

However, later Richards said the shaft was “completely full of rockfall and landslide” and they “couldn’t identify any way” down it. “We’re not entirely sure what the next steps are,” he said in a video clip on Monday.

In the early hours of Tuesday, Bongkawong said in a Facebook livestream that the team was resting in order to resume operations in the morning. The Thai diver said he was sleeping outside the entrance of the cave.

Rescuers were expecting to rappel back into the cave again and resume pumping out water on Tuesday, according Finnish diver Mikko Paasi.

Water is a constant threat

The men became trapped during an expedition to find gold within the cramped and muddy underground caverns. 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.”

Heavy machinery was brought into dig more water retention ponds, to hold water pumped from the cave.

Inhis livestream, Bongkawong expressed optimism at being able to rescue the remaining two trapped villagers, but described the challenges of operating during monsoon season.

“To go down into these holes, to go down inside the cave, if suddenly it rains, dumping down some water, it’s like pouring water into a straw,” he said. “Instantly, the straw will be filled up. You can’t get in. You can’t get out. That’s something we need to be mindful about and it’s the biggest risk.”

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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