Some types of used plastics are very stable.
They also contain a significant amount of carbon.
An example is polyethylene (PE).
When waste is collected for recycling, not every plastic part can be reused. There is a fraction that is usually burned, creating CO2.
We want to avoid accumulating CO2 in the atmosphere, and look for methods to store carbon under ground.
Why don't we store used plastics under ground, in a long term stable way?
I am not sure how normal mixed waste dumps behave in the long term, but they have drawbacks. For example they can leak substances from the other parts of the mixed waste and they can burn.
But other methods seem stable to me: Solid PE blocks of standard pallet size, stacked underground in abandoned mines should be long term stable, I think. But it is not about the specific place of storage, a mine is only an illustration to argue that it is possible.