Why don't we store carbon by storing used plastics under ground?
Some locations don't have the available space to dump rubbish underground and those that do are running out of suitable dump locations.
Similarly, the location of abandoned mines and the available dumping space is also limited. Prior to using abandoned mines, many will need to be made safe by implementing ground control measures to prevent injuring from falling rock and to prevent cave in.
Some mines, particularly coal mines have been mined to subside shortly after coal has been removed, making such mines unusable.
Some mines were constructed underneath the water table and are now flooded. Such mines would need to be de-watered and made safe. The water would need to be dumped somewhere, which and may need to be treated prior to dumping because of metals that may have leached into the water from minerals in the abandoned mine.
Once such mines have been backfilled the ground water will again flood the mine. The question of how any plastics would behave when submerged needs to be investigated to mitigate against deleterious environmental effects.
Some old mines were established using manual methods and have short height and narrow width tunnels which would require a lot of manual handling to make the mines safe and to dump plastics in them. Alternatively, the tunnels may need to be enlarged to allow the used of heavy machinery in dumping the plastics. This would be expensive and may not be practical in some cases.
The location of some mines is far from population centers which would required expensive haulage to the mines.
It would be to reuse plastics in another way, if possible, or to stop using them or significantly reduce their usage.