HONG KONG — Rescue workers searching flooded caves in Laos reported hearing a “knocking sound” from inside the system as they sought to extricate two people who have been missing for nearly two weeks.

A group of seven villagers entered the caves in Xaisomboun province on May 19 to search for gold, but were trapped there by flash flooding following heavy rain.

Five villagers were rescued last week — one of whom was taught to scuba dive in order to make his way to the mouth of the cave. The rescue team was able to pump enough water out of the system for four more people to crawl out on their own.

Rescuers are now exploring a sixth chamber where the remaining two men might be located, with one of the coordinators reporting in posts to Facebook that workers heard a “knocking sound” seemingly responding back to their own knocks. 

“Yesterday, when we knocked, there was a signal responding back. It was a knocking sound meant to be heard. Based on our initial assessment, this is considered not to be a reflection or an echo of the sound,” Kengkaj Bongkawong — the head of the Thai group Metta Tham Rescue Kalasin, which is taking part in the operation — wrote on Facebook.

“Moving forward, it won’t just be a matter of sitting around waiting for water to be pumped out,” Bongkawong added. 

Workers are using LiDAR data and satellite imagery to scan the area and assist in rescue efforts, Bongkawong said. “We obtained a lot of useful data to verify the rock layers and analyze the mountain structure to locate cave chimneys, vertical shafts, or air holes that might have a chance of connecting directly to the chamber where they are trapped,” he wrote.

The villagers who were rescued last week were immediately treated at the entrance of the cave. One man who was rescued on Friday was taken to hospital.

Divers from around the world flew out to the site to help with the rescue efforts. Officials said the caves are so tight that they needed thin divers to successfully navigate the space.

Australian cave diver Josh Richards, who was helping with the rescue operation, told ABC News of the successful recovery of the first villagers, “I was literally about to head into the cave myself when all of a sudden we heard all these cheers and spun around, and four very muddy miners just suddenly emerged out of the out of the cave on their own.”

The cave is a highly restrictive, fractured network of tunnels and the men were located more than 800 feet from the entrance, down a steep 45-degree subterranean slope.