The rescue operation resumed on Saturday after a break in the heavy rain eased flooding in the system of caverns in Tham Luang.
Thai authorities are hopeful that the group are still alive and have found a dry place within the cave to await help.
“Today the situation is much better and we will be here all night,” said the governor of Chiang Rai province, Narongsak Osatanakorn.
A medical evacuation drill was held to see how long it would take to get rescued people out of the cave, into 13 ambulances and to the nearest hospital.
Elite Thai Navy Seal divers are being assisted by experts from a British cave exploration club, a Chinese disaster response team and military personnel from the US and Australia.
They have been repeatedly forced to suspend the search because of high water levels inside the cave complex despite attempts to pump it out.
Efforts have also been made to find shafts on the mountainside that might serve as a back door to the blocked-off areas where the missing boys might be sheltering.
“Yesterday our team climbed into one shaft, and went in around 50 yards,” said National Deputy Police Chief Wirachai Songmetta.
“Today we will re-enter the second chamber that we found and try to find passages that could lead to other chambers.”
Survival packages containing food, equipment and a map of the cave were dropped into the shafts in the hope the boys might find them.
The boys, aged 11 to 16, and their 25-year-old coach Ekapol Chanthawong entered the Tham Luang Nang Non cave after a soccer game on 23 June.