calculate depth in unity 3d

I am trying to create an app where I need to show the oil drilling process and for that i need to calculate drilling depth from earth’s surface to deeper down the oil reservoir in feet i.e. 1000ft-9,000ft
Along with drilling pipe drilling down the hole need to show the depth in feet while the drilling processes is going on…

you can use the euclidean distance between the object’s position who represent your reservoir and the position of your drilling. And display the distance after a scaling to fit with your length scale