A Gravimetric Mine is an advanced piece of Directorate interstellar warfare technology, and is one of its most important weapons in effectively containing - if not stopping - the Mortis threat. Born out of the Directorate's unrivaled mastery of FTL technology, these Mines are one of the few devices which the aliens have little if any ability to replicate or counter.
A Gravimetric Mine is a very large, vaguely spherical, automated construct - a sort of space station that uses ion thrusters to maintain a constant position or conduct slight positional adjustments. The Mine is studded with a number of Tannhauser-Orpheus projectors that allow it to emit beams of highly concentrated pseudo-gravity, effectively stopping ships in their tracks. It is also wreathed in a bubble field of super-dense pseudogravity that crushes most any kind of matter at a molecular level, rendering explosives inert and harmlessly refracting light, making the Mine impossible to destroy with anything except a Directorate Neutron Field Missile (which has its own high-gravity field and can "plow" right through the defensive bubble of the Mine) - a technology which the Mortis are not capable of replicating. The Gravimetric Mine may also expand its own bubble to cover a wide area of space, though this cannot be done in close proximity to a planet for obvious reasons.
The Gravimetric Mine, similar to the aquatic mines of old, is primarily a deterrent rather than an offensive weapon. It is capable of ripping enemy ships out of FTL transit, stopping them from making a "jump" to retreat from the system, and physically trapping them to hold them in place. Smaller ships will eventually be pulled into range of the Mine's gravimetric field and torn to pieces, while larger ships will be rendered effectively immobile. Once a Mine's beam has latched onto a warship, it is effectively incapable of firing up its own gravimetric drives, making rapid sub-C maneuvering and FTL jumps impossible, while the force exterted by the beam holds the vessel in place or draws it ever closer to the Mine. Directorate warships can avoid being snared by their own mines by firing up their Tannhauser-Orpheus fields while still outside the Mines range and pushing through the fields, but Mortis gravimetric technology is too weak to achieve the same.
These Mines are usually seeded around Directorate worlds to stall and trap attacks. By doing so, the Directorate forces the Mortis to proceed in a linear, planet-hopping fashion: attacking from planet to planet in a chain, because any fleet sent to attack deep into Directorate space would be trapped by Gravimetric Mines around a particular star and pounced upon by reinforcements.