When observed from their summits, the shadows cast by the world’s highest mountains are almost always perfectly triangular, regardless of the mountain’s actual shape. This perspective effect is caused by the fact you are looking along a long tunnel of shadowed air, which the finite size of the sun causes to converge and taper away in the distance. As the shadows of the mountains can be hundreds of miles in length and as the top is so far away from you, any irregularities in the peak are lost and we are left with the impression of a perfect pyramid.
Luckily for us this natural phenomenon results in some truly breathtaking images.