To accomplish this feat we must remove the air from the inside of the barrel so that it no longer balances the pressure from the external air.

To remove the air, we'll put a small amount of water in the bottom of the barrel.

Gas burners heat the water to boiling, and steam from the boiling water gradually drives the air out of the barrel.

When all the air is replaced by steam, the barrel is capped tightly.

Now we'll cool the barrel by loading ice onto the top.

As the steam cools it condenses to liquid water, which falls to the bottom of the barrel.

The steam pressure inside no longer balances the tremendous force due to external air pressure, and the barrel is crushed.