A graphic of a multi-kinetic neutronic mine
The multi-kinetic neutronic mine was a weapon of mass destruction utilized by the Borg. The mine had a yield of five million isotons, enough to affect an entire star system. Shock waves from the mine would spread over a radius of five light years. In 2374, Seven of Nine proposed the neutronic mine as a means of delivering modified nanoprobes to Species 8472, noting that it would be "efficient". However, the fifty trillion nanoprobes necessary to arm even a single mine were well beyond USS Voyager's production capabilities given the time constraints, not to mention Captain Kathryn Janeway's ethical reservations about such a device. (VOY: "Scorpion, Part II")
Background
Curiously, the shape of the mines resembled that of the rogue Borg ship from TNG: "Descent" and "Descent, Part II". The size of the mine could be considerably smaller, however; taking the size of a nanoprobe to be approximately equal to the size of a red blood cell, 50 trillion nanoprobes take up considerably less than 1m3. It is often speculated that therefore, the display is actually of the minelayer.