Crevasses are deep, wide cracks in the ice. Sometimes when the wind blows, snow will accumulate over the crevasse making a snow bridge. This bridge is deceptively dangerous because it will not hold your weight. If you step on a snow bridge, you will fall into the crevasse.
Once inside the crevasse, the heat from your body will melt the ice causing you to become wedged. In some cases, people have been successfully rescued from a crevasse. Most of the time, the victims freeze to death before being rescued. The Queen knows of one instance where the frozen body came out of the crevasse in pieces; a grappling hook had to be used to retrieve the remains.
The pick axe is used to test for these snow bridges. If the crew is unsure about a particular spot, an auger will be used to test the depth of the ice, as shown here.