The loss of the Vulcan cruiser Seleya in the Delphic Expanse, whose crew were driven violently insane by exposure to trellium-D and whose wreck Enterprise found in 2153.
The Seleya was a Vulcan ship on which T'Pol had once served. It was sent into the Delphic Expanse and never returned, one of the reasons the Vulcan High Command warned Archer against going there.
In the Enterprise episode Impulse the NX-01 finds the ship adrift in an asteroid field rich in trellium ore. The Vulcans had tried to line their hull with it for protection against the Expanse's anomalies, not knowing that the substance destroys the neural pathways with which Vulcans control emotion. The crew had become savage and beyond treatment.
T'Pol was affected during the boarding and turned on Archer before Phlox treated her. The Seleya was destroyed, and Enterprise could not use trellium-D while T'Pol remained aboard.