Why mine clearance continued long after both world wars

How did mine warfare change across the world wars, and what did that mean for cleanup?

The wars left enormous fields requiring years of clearance; most usable routes were reopened, but scattered mines and munitions remain, especially in the Baltic.

Both world wars involved liberal use of naval mines, leaving extensive clearance work afterward. German crews in trawlers formed a substantial part of that effort. Clearing navigable areas did not remove every weapon from the seabed. The Baltic's remaining mines and munitions still make seabed searches hazardous, because an object discovered during a survey can be dangerous to disturb.