But on this Saturday, they did not come out, and authorities now believe the young people may have been trapped by rising water from a heavy rainfall that flooded a stream near the entrance.
"The team went down to a depth of 5 meters (16 feet) and found a large chamber. but we've found no trace of the children", the navy unit said on its Facebook page.
Park officials and volunteers search for 11 football players and their coach believed trapped in Tham Luang Nang Non cave in Mae Sai district, Chiang Rai, on Saturday.
Footage on Thai television showed bicycles, backpacks and soccer cleats left outside the entrance to the cave. The chamber is about 4 kilometres (2.5 miles) from the entrance of the cave, which is thought to be about 6-8 kilometres (4-5 miles) long.
The search began around midnight on Saturday after police received a report of a missing child from a mother who said her son had not returned from the soccer practice that evening, ThaiPBS reported. "I believe they're all still alive but they might be exhausted. we should get good news today", Rear Admiral Arparkorn Yookongkaew told Reuters. He said the fact that they were athletes should help them endure the situation.
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The derailment also caused concern downstream, including as far south as Omaha, about 150 miles from the derailment site. Metropolitan Utilities District said it was monitoring pumps it uses to pull drinking water from the Missouri.
"We've preparing everything, water pumps in case the water is still high when we find them and food and drink", he said. Murky floodwaters were also hampering the effort.
The cave, cut into a mountainside in far northern Thailand near the border with Myanmar, is a local tourist attraction but can flood severely during the rainy season, which runs from June to October.
Kamolchai Kotcha, an official at the forest park where the cave is located, said on Monday that attempts to reach the chamber had failed as the passage was extremely small, "flooded and covered with sand and mud".
Local media reported that the team is believed to have entered the cave after a practice session, before a flash flood trapped them in the rock structure as rising waters blocked their exit. The publication said the water inside had reached levels in some areas of 20 feet.
Anukoon Sorn-ek, a geographer and expert on the Tham Luang Nang Non cave, said divers have made it about 3 kilometers (1.8 miles) into the complex.