Authorities say the boys, aged between 11 and 16, and their coach entered the Tham Luang Nang Non cave in Chiang Rai province on Saturday and have been seen since then.
Reports say the group were trapped after heavy rains flooded a stream at the cave's entrance, a tourist attraction which runs for several kilometres underground.
Bangkok Post newspaper reports that, visitors must cross a small stream to enter Tham Luang Nang Non, making it inaccessible if the stream floods.
Police Colonel Komsan Saardluan told news agency AP that the cave can flood up to 5metres (16ft) during Thailand's rainy season, which runs from June to October.
Rescue workers began searching for the group on Saturday night after they were reported missing. They found bicycles along with sports equipment outside the cave.
Defence Minister General Prawit Wongsuwan said divers from the Underwater Demolition Assault Unit of the Royal Thai Navy entered the cave on Monday.
Courtesy of the BBC News.