Schadenfreude

Each doorway to each building in my part of Berlin has right in front of it a hole in the ground about one foot square and one/two feet deep, covered by a grate. This is positioned exactly where your keys would land were you to be unfortunate enough to fumble as you try to open your door and drop them. Said holes can only have been placed there for this reason. Nothing but keys goes into or out of these holes, and there can be no other conceivable reason for their existence.