Why is it extremely dangerous to put dry ice in a closed container?

Dry ice sublimates (turns from solid into vapor) in natural pressure conditions: 1 atmosphere, 101.3 kPa, or 14.7 lbf. This can be seen by the phase diagram of carbon dioxide (see link)

From there it follows the universal gas law: PV=nRT. The middle part of that equation can be ignored for simplicity. Pressure is directly related to temperature. At room temperature, the pressure of CO2 is 853 psi. Unless you have a very strong container, the first answer to this question is correct...

because it will blow up!!! on mythbusters they did this in a pop bottle and it was a really big boom!