Created over millions of years, fold mountains form the largest peaks on Earth.
But most began life as little more than mounds under the ocean.
Fold mountains are formed gradually by powerful movements of the Earth's crust.
Extreme pressure between tectonic plates folds rock, forcing it upwards, and these gradual folds add up to create a steeper and steeper slope.
The degree of inclination
Or the rate of ascent and descent
The slope of a straight line is calculated by dividing the change in vertical height by the change in horizontal distance.
At first, the ocean bed has a slope of zero - it's flat.
Despite moving only a few millimeters each year, folds develop over time, and a slight slope of 0.1, or 10%, builds up.
Over millions of years the fold mountain continues to grow and, as the slope gets steeper, it rises above sea level.
Over time it will grow into a formidable peak - climbers must scale a slope of 0.4.
Eventually, they reach the apex.
Apex - the highest point from the base
On the other side, climbers must descend down a similar face, but this time with a negative slope because the slope is descending.
Some fold mountains also have sheer cliff faces with straight drops.
Because their vertical distance would be divided by zero, these gradients are said to be "undefined" or "infinite."
Even though climbers can measure the slope of mountains, they can never be quite sure of their calculations.
Because each time the Earth moves, fold mountains continue to grow.