By Rebecca Wright, Kocha Olarn, Angie Puranasamriddhi, June Jeong, Laura Sharman, CNN
Rescuers are focusing on a newly discovered cave shaft.
Rescuers in Laos are racing against time as they investigate 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.
