This is an important monument and the understanding of our place in history. We who love Megalithic monuments would be grieved if an untrained operator ruined the monument by accident. This could not be repaired afterwards.
It really needs for the size of the cellar to be curtailed and a surface boundary to be agreed with the management of the monument.
Of course if the project could be cancelled and moved to a better (brownfield) site where an underground cellar would be an improvement to the area that would be ideal. Maybe a garden on top?
Something is not right. This message is just to keep things from messing up down the road