They mine and countermine, until it isn't safe to predict who is immune and who isn't.
Countermine:—As before, but moored 29-1/4' below the surface.
It is not an easy matter to decide just where the countermine should be sunk or how far to go.
I know where the mine is to be sprung; and I will countermine!'
They mine and countermine expeditiously in the Channel Fleet.