[Fredgoblu]

Won't there be a big risk of an explosion of CO2

Ummmm...you might want to check your Chem 1 book next time. You will (probably) be surprised to know that it can be found in certain models of fire extinguishers.

[RandyRep]

Right. I knew that.

But late at night, I used the word "explode" to mean a pressure-generated burst (through some sort of path to the surface) into the atmosphere rather than a chemical reaction burst.

The way the original report was written, it sounded like they were going to pump caves and underground vaults with CO2. A later post said that was quite common.