The boys' bicycles and soccer boots were found at the mouth of the Tham Luang caves, in the northern province of Chiang Rai, after they went missing late on Saturday.
They were believed to have been trapped by rising waters inside the cave network when heavy rain hit.
Members of a navy "seal" unit including a team of divers, are searching the cave system.
"The team went down to a depth of 5 metres (16 feet) and found a large chamber ... but we've found no trace of the children," the navy unit said on its Facebook page.
A navy commander overseeing the search said he was hopeful the 12 twelve boys and their 20-year-old coach would be rescued.
"I believe they're all still alive but they might be exhausted ... we should get good news today," Rear Admiral Arparkorn Yookongkaew told Reuters.
People wanting to explore the cave are meant to get permission. Thailand is in the middle of its rainy season when caves can flood.
(Reporting by Panarat Thepgumpanat; Additional reporting by Amy Sawitta Lefevre and Juarwee Kittisilpa; Writing by Amy Sawitta Lefevre; Editing by Robert Birsel)
