The volume of "likely tunnel voids" in Silbury is 310m3. So the volume of the Merewether tunnel is likely to be something around 150m3 or less I suppose.

The volume of Silbaby is one third of pi r squared timed height, minus about a tenth for the flat top maybe? 8 metres high? 14 metres radius? Which is 1500 m3, ten times greater.

(Please check this Steve…)