Navigating this hard terrain, which is also pitch black and may involve struggling against fast-flowing, muddy water in some areas, will be hard.
Tim Newton, a journalist based in Phuket, told CNN it will take time to get the boys out of the cave.
Experts say divers have required three hours to reach the boys, located about 4km from the mouth of the cave.
The boys range in age from 11 to 16, and are with their 25-year-old coach. It is believed they entered the cave when it was dry and sudden heavy rains blocked the exit.
When the group will be able to leave the cave isn't known due to flooding and other factors that could make their extraction unsafe.
"God bless you", another member of the rescued party could be heard saying.
The boys showed glimpses of smiles throughout the brief video, which was recorded sometime Tuesday and posted to Facebook on Wednesday.
"How many of you are there - 13?"
The soccer team was found deep within the cave on Monday night, and Thai authorities say while they want to get them out as soon as possible, they'll only attempt to move them when it's clear they're strong enough to leave. "You have been here 10 days". We are coming. OK?
Night's sister said her brother and the three boys from the same village would gather at their house almost every day before going out to play. The rescuers had to fight a current as they pulled themselves through narrow, flooded passages by gripping the walls.
The rescuers, lead by British men Richard Stanton, Robert Harper and John Vonlanthen, discovered them around 400m from an area of the cave known as Pattaya Beach. Well, that's just grouse - and it's what Thai authorities reportedly have planned for the kids holed up inside the Tham Luang cavern. "We have to see they are ready", he explained.
The priority is to get the team's strength up before they start the tricky journey out, officials said, reluctant to offer a concrete timeline.
Volanthen told the Sunday Times in 2013 that cave diving is not the pursuit of those who crave thrills. "Panic and adrenaline are great in certain situations - but not in cave diving", he said.
A limestone cave complex is like a giant sponge, said Amy Frappier, a professor of geosciences at Skidmore College who has done extensive research in caves. Newly-released videos appear to show the boys in good spirits, smiling as they interact with divers and medics.
But there were no exact details on how they were going to be rescued, given that the cave system was still flooded.
He said it was unknown when an extraction could be attempted, but it was unlikely to be today.
The boys and the coach are no longer alone.
After days of solitude, the boys are now receiving a string of visitors, including rescue divers and health professionals and they are being fed liquid, high-protein food. The British Cave Rescue Council, which has members taking part in the operation, estimates the boys are around 2 kilometers (1.2 miles) into the cave and somewhere between 800 meters (half a mile) to 1 kilometer (0.6 mile) below the surface.
Experts say diving out is laden with risk - more so as the boys have never dived before and some may not be able to swim.
Benjamin Brown is a reporter for Fox News.
Medical personnel were providing medicines and food while experts assessed conditions for getting them out safely, a task that the government said would not be easy. "In these 10 days, how many million seconds have there been?"
A final rescue option could see the boys lifted through the roof of the cave chamber to safety, either through a natural opening or a drilled entry point.