An explosion of a Klingon moon that damaged the Empire.
Praxis was a moon of the Klingon homeworld Qo'noS and the Empire's key energy production facility. In 2293 it exploded, an accident blamed on over-mining and poor safety, and the USS Excelsior under Captain Sulu was hit by the shock wave while on a survey mission. The opening of Star Trek VI: The Undiscovered Country shows the blast.
The explosion polluted the homeworld's ozone layer, and Spock estimated that Qo'noS had about fifty years before its atmosphere was depleted. With the Empire unable to afford both its military and the recovery, Chancellor Gorkon reached out to the Federation for peace, and Spock volunteered Kirk to escort him to talks.
Kirk, who resented the Klingons for the death of his son David, accepted reluctantly. Gorkon was assassinated by conspirators from both sides who wanted to stop the peace, and Kirk and McCoy were framed for the murder. The truth came out at Khitomer, where the peace talks led to the Khitomer Accords.