Gravitic Mines

Gravitic mines are a thought by most to be remnant of the great war. They are built to be almost impossible to detect until they become active, they can lie in wait for centuries before a ship passes close enough to trigger one.   The common rumours say they are thought to have been deployed by automated minelayers that self-assembled these mines from raw resources, as such it's unknown exactly how many exist, but estimates suggest that there may be as many as billions of them still remaining in the Prime system.   What makes them difficult to remove is also what made them so effective: They are each made with slightly different programming and are often armed with different types of payloads.  

"It's been reported that some mines will attempt to quietly attach to the hull of a passing ship, exploding days or even weeks later. Some mines have been reported operating in groups, activating nearby mines and using a 'pack hunting' mode to box in targets. Yet other mines apparently convert all their explosive output into a single focused blast of energy similar to a laser blast of immense proportions.   The one unifying factor is that they all rely on a detection method that relates to gravity displacement - thus, even a ship 'cold coasting' nearby can still be detected."