Four boys were brought safely out of the cave on Sunday.
But the mission was paused overnight for air tanks to be replaced.
Rescuers decided to go ahead with the hazardous operation because of fears of rising waters. The group have been trapped since 23 June.
Activity at the dive site early on Monday suggests that the operation to free the remaining members of the group may be under way or about to resume.
At least seven ambulances have driven towards the cave entrance.
Rescuers took advantage of a break in the rain on Sunday to launch the mission earlier than some expected.
The first stage of the mission ran "smoothly" and the rescued boys were in "good health", according to the Thai authorities.
But with heavy rainfall overnight and more forecast for the coming days, divers will need to work quickly to free those still trapped. On Saturday, Narongsak Osottanakorn, governor of the Chiang Rai province, said that teams had a three to four-day window to carry out their operation.
A team of 90 expert divers - 40 from Thailand and 50 from overseas - has been working in the cave system.
Wearing full-face masks, which are easier for novice divers than traditional respirators, each boy is being accompanied by two divers, who also carry his air supply.