Why should it be assur to put your fingers in the cracks – it is definitely under ground level, as bedrock continues over fifteen meters above, so at the worst it should just have a din of michilos, which don’t have kedusha?
Oh, and lesschumras (or should it be chumros?) – please stop stating things as blatant fact when they are actually speculation – there may be some supporting evidence, but it is still just speculation – in particular when, in fact, there are many rabbanim who are cholek.