If they're under water they'll probably decay to methane instead of CO2 which is even worse. If you can put them somewhere dry, though, that might work. Bury them in the dessert maybe?

It has been suggested. Compress gigatons of them them to make something that won't float, and dump the result in a deep mid-ocean trench.

If carbon-free energy were money-free...

My third grade idea for trapping CO2 was to fill upside down buckets with gas and put them push them to the bottom of the ocean. The pressure would eventually compress the gas into a solid, so it would sink and remain there.

I'm still accepting funding in the from of rare stamps.