Soldiers from 3 Rifles have been pushed to the "point of failure" on Exercise Cairn Patrol – a gruelling section patrol competition.
The exercise sees the troops travel over a 40km course crossing several peaks, from the Scottish borders back to their base in Edinburgh, while each carrying 25kg on their back.
Captain Rhys Evans, 3 Rifles, told Forces News: "We're pushing the rifleman and the commanders to the point of failure but within a controlled environment."
One of the most challenging parts of the exercise is the wet gap crossing.
After hours in the hills, the eight-man sections had to cross 80 metres of five-degree water against the clock.
Corporal Regan Brown said it was the first time he had ever attempted a wet gap crossing.
"It's quite different to actually get in the water and actually conduct it," he said.
"It was a good experience but quite challenging knowing how cold the water was."
Despite the soldiers' training, crossing water is still dangerous and diving experts from the Royal Engineers were drafted in to keep an eye on them.
Sergeant Andrew Lumley, 26 Royal Engineer Regiment, explained that a maximum of four people were allowed in the water at one time, with only one weak swimmer among them.
"I have a safety boat which needs to keep an eye on the weakest swimmer, if any of the strong swimmers get hit, obviously the safety boat will have to react to them as well," he said.
Throughout the exercise, the personnel faced a number of challenges designed to test their skills including "communications, medical, vehicle recovery [and] recce", Capt Evans said.
The troops were then awarded points based on how they performed during the tasks, which were used to decide the overall winner of the competition.
While these vital skills are fairly straightforward on their own, after spending all night out in the Pentland Hills, fatigue does play its part.
Lance Corporal Dylan Johnson-Williams said he found the patrol "really challenging".
"I thought because I am really good at fitness... I thought that I wouldn't struggle but, obviously, I was kidding myself," he said.
"Probably about 12 hours in, [I] definitely started feeling it."