A filled harmonic resonance chamber.
The harmonic resonance chamber is a device designed by the Borg to contain and stabilize the Omega molecule. However, the Borg never constructed one because they were unable to synthesize more molecules.
Seven of Nine supervised the construction of a chamber aboard the USS Voyager in 2374, after Voyager discovered a large number of Omega molecules. She modified the chamber to emit an inverse frequency, that would dissolve the molecules' interatomic bonds.
Taking inspiration from the aliens who synthesized the molecules, Seven believed she could stabilize Omega by calibrating the chamber to the natural harmonic frequency of the molecules. However, Captain Kathryn Janeway decided that the risk of Omega destabilizing was too great, and ordered the molecules neutralized as planned. The chamber successfully destroyed most of the molecules, and the remainder were eliminated when the chamber was detonated by a modified gravimetric torpedo.
Shortly before the chamber was ejected from the cargo bay, Omega stabilized spontaneously and Seven of Nine witnessed what Borg describe as 'perfection'. (VOY: "The Omega Directive")