Imagine you have a 55 gallon drum that's completely airtight. You connect a hose to it and start pumping in air. Assuming all connections are stronger than the drum, eventually the drum explodes.

Now, instead of pumping air into the drum, what would happen if you started sucking air out of the drum? Eventually the air pressure outside of the drum would be greater than the pressure inside the drum and it would crush, or implode.

You can watch a short video here.