Re: Anti-submarine warfare
 

I meant that these are weapons in the BM series (= Bombenmine) rather than the Fl.W.B. Wasserbomben which were, apparently, purpose-built for ASW.

Re: Anti-submarine warfare
 

I would say that it is a mine whose self-destruction system will detonate it if it falls on soft ground or too shallow water, due to the tidal effect, it can become visible during low tide, allowing the opponent to explore its secrets, or on hard ground.