Klendth was a planet where the Loonkerian outpost was located.
Sometime prior to 2286, a piece of bioengineering technology, known as the Universal Atmospheric Element Compensator, was made on the Loonkerian outpost on Klendth. this marked a revolution in terraforming. the device and the process used on the planet Klendth vastly increased the speed that terraforming could be conducted.
Spock was asked about this technology in his memory test on Vulcan following his "rebirth" on Genesis Planet several months earlier. (Star Trek IV: The Voyage Home)