The argument was that before we drilled holes into them, those stone formations had held similarly sized pockets of natural gas for eons, so just refilling them with CO2 would be fine. It sounds not completely stupid on first thought.
On second thought it sounds completely stupid tho.
I gave my kid my big crate of capsela a few years ago. Aside from having to sand a few contacts it all worked great after 25 years of non-use and also led us into some cool 3d printing projects. I wish they made more toys like this today.