Crabtree persistently suggests considering the underground people theory of female explorer Elva Gordon, whose deep cave expeditions were aborted by the scientific community and financier, yet advances her banned hollow earth theory at a public lecture Murdoch attends with Julia. William searches regular suspects for a wave of shop - and bank robberies from tunnels which are collapsed again almost trackless, remarkably focused on stealing diamonds. He scientifically works out why those, what kind of machine can dig that way, in what range and how to trace it with seismic trigonometry, to catch the non-mole culprits.
—KGF Vissers