Divers found bicycles, shoes and footprints in the five-kilometer-long Tham Luang cave, where the youngsters' soccer team and their coach have been trapped since Saturday, an official told AFP.
The group is believed to have retreated into the cave as heavy monsoon rain fell and flooded the entrance of the cave in a national park near the border with Myanmar.
"We are surveying the areas around the mountain and will try to find another exit or entrance," Chaing Rai provincial governor Narongsak Osottanakorn told AFP, adding that hundreds of officials have been sent to help with the rescue.
But one expert who has studied the cave for the past six years told officials there was only a single entrance, which is now blocked by water.
The team, aged between 11 and 16, went into the cave after soccer training on Saturday. One of their mothers raised the alarm when her son failed to return home.
Narongsak said rescue operations would continue overnight and teams were on standby with supplies for the trapped children. "We've preparing everything, water pumps in case the water is still high when we find them and food and drink," he said.