Dont take it too literally. Cave maps, paticularlythose under water, are hand-drawn. They cover key features needed for navigation but are not considered accurate for dimensions. Travel time means far more than accurate distances where underwater currents get involved. This is a large part of the digging problem: lining up the eyeballed drawing with an exact spot on the surface.

Cross-section: https://twitter.com/JamesWorldSpace/status/10151317727433687...

Another version: https://www.thetimes.co.uk/imageserver/image/methode%2Ftimes...