Carbon sink

The Extinction people often tell us that there is too much carbon in sky and that there is too much plastic in the sea. So perhaps one of them can explain the following.

Plastics are mainly made up of carbon, hydrogen and oxygen, with the proportions varying from plastic to plastic, it being a very large family. In the case of cellulose - also a big ingredient of trees - the proportions are 6:10:5. Or put another way carbon and water bound together, ultimately by the power of photosynthesis, by the power of the sun. A giant heat sink.

So rather than getting into a panic about plastics, why can we not just bury them in holes in the ground, if necessary deep holes? That way the carbon gets trapped in the ground, in much the same way as it might otherwise be trapped in tree trunks, lasts more or less for ever and does not find its way into the sky or the sea. Maybe better not to have so much of the stuff in the first place, but given where we are, what is so difficult about burial?

PS: in the snap above of a chunk of a cellulose chain, taken from Wikipedia, I think that carbon (C) is black, making up most of the rings, hydrogen (H) is white and hydroxide (OH) is red. The only catch being that it does not seem to add up to the desired 6:10:5; rather to 6:12:5. Clearly some biochemical mystery I am not privy to.