> surely the Reg should have a good unit of depth?

I would suggest for small measurements, the PH or Pot Hole, being 2" deep, would be suitable. (Apparently Councils have been recommended that any hole that is less than 2 inches or 50mm deep is no longer a "priority" for repair)

For greater depths, the "Blackburn (Lancashire)" or Bbn(L), being 4000 x 2" holes would be suitable.

Thus the average depth of the Grand Canyon would be approximately 8 Bbn(L) and the lunar craters around 32Bbn(L)

HTH.