Praxis
Praxis was a Klingon moon. It was a key energy-production facility for the Klingon Empire until 2293.
That year, a combination of mishaps occurred, causing an explosion that disintegrated most of the moon - the disaster was later attributed to "overmining and insufficient safety precautions."
A Subspace shock wave from the incident disrupted operations aboard starships, including a Starfleet ship, the USS Excelsior. The result of that explosion severely damaged the ozone layer of Qo'noS, leaving it with approximately 50 Earth standard years of oxygen left. With the Klingon Empire's economy and infrastructure in tatters, peace negotiations were established with the Federation that same year. This tenuous peace anticipated the later alliance between the Federation and Klingon Empire. (Star Trek VI: The Undiscovered Country; VOY: "Flashback")
Background
It has not been established as to how the Klingon homeworld of Qo'noS managed to recover from this incident. It is possible that with the established peace between the Federation and the Klingon Empire, scientists from both parties worked to fix the situation.
