Lantaru Sector Research Station destroyed by Omega molecules.
The Lantaru Sector Research Station was a classified Federation research station during the late 23rd century in the Lantaru sector, led by Professor Ketteract, working towards creation of a new inexhaustible source of energy.
When in 2269 the scientists were able to synthesize a single molecule of the source, called Omega, it destabilized after a fraction of a second. The enormous amounts of energy released by the molecule breakdown destroyed the research facility, killing all 127 scientists in it. The explosion also severely disrupted subspace in the sector, making it impassable to standard warp drive travel.
The accident and its consequences made Starfleet realize that only a few molecules could end warp travel throughout an entire quadrant or galaxy, which led them in 2270 to enact the classified Omega Directive.
(VOY: "The Omega Directive")