What are the Thai cave rescue divers up against as push to pluck remaining boys and coach from underground looms?

A dive expert currently helping at the scene of the trapped soccer team in Thailand has given an insight into the conditions rescuers are fighting against to help the trapped boys.

Early this morning four boys were rescued from the cave but eight others and their football coach remain underground pending the resumption of the rescue operation scheduled for this afternoon.

Danish volunteer Ivan Karadzic owns a diving school in Koh Tao and told the Sydney Morning Herald that conditions underground in the Tham Luang Nang Non cave are treacherous.

The boys are thought to be at least 2km deep into the cave, which has at least five sections where underwater dives are required to navigate through narrow passages.

Dive tanks are needed on the journey in and out - the average tank holds about 2200 litres of air, and a skilled diver uses between 15-20 litres per minute near the surface, with that amount increasingly rapidly at depth.

That adds up to about two hours of air per tank - meaning multiple tanks must be hauled by rescuers trying to reach the boys.

Trained divers are taking about five hours to get from the cave's entrance to where the boys are trapped - one way.

There are a number of very tight squeezes along the way, where tanks and equipment must be removed, pushed through the gap first, and then the person can squeeze through.

Rescuers also need to be mindful of the air quality in tight sections of the cave - too much carbon dioxide buildup would be deadly.

There are strong currents under the water and visibility is almost nothing in the muddy depths.

In addition, there are mineral deposits, stalactites and stalagmites throughout the caves which can be very sharp and can pose a danger both to people and equipment.

There is a one-kilometre section of the cave which is walkable, but the ground is thick, sticky mud under the surface of floodwater, which slows down and fatigues rescuers significantly.

A significant amount of water has already been pumped out of parts of the cave, but there is just too much to take out completely, plus the monsoon rains are refilling the cave.

Some have suggested drilling down from above to reach the boys - they are estimated to be about 1km below ground and the cave walls are thick rock.