Mushroom cloud. The geometric lines?
The geometric "straight" lines that form the outline of what looks like the silhouette of the tent, that is in the foreground of the shot, at the bottom of the image. You can see the corners of the back of the tent. Just to the right of the back of the tent, there is a person standing up. They are half in and half out of the image. The top of the tent lies slightly below the shoulder. The person has their back to the camera. They are wearing a coat with a hood. The hood is down. They are also wearing a hat. The top of the hat is dark and there is band of lighter material around the circumference of the hat, the person is looking at the mushroom cloud (as you would probably expect). Although the image seems damaged, you can see the darker tent as you move from right to left along the bottom of the image. Only the top half of the tent is in the image. There is more damage to the image on the left hand side, but you can see the change in brightness as you get to the front of the tent. The tent is fairly parallel with the respective to the image, which means the mushroom cloud is over the summit and probably behind it to some extent on the South is the mountain and up wind more or less. Based on the scale presented by the size of the person in the image, the dark outline of the tent is about 4 metres long - which is about right.
You will need to zoom in to the image to see it more clearly. I would suggest you look at it in a darkened room too.
The mushroom cloud still appears to have incandescent gases in it which creates a big contrast difference . The original image is also very under exposed for the light conditions and damaged in places. Not sure if the cloud has smoldering debris falling from it or if this is just damage to the chemical film.
If I'm right then this image could show the beginning of the events that led to the death of the hikers.