From Bulbapedia, the community-driven Pokémon encyclopedia.
Mount Quena (Japanese: ピュアズロック Pures Rock) is an anime-exclusive location in Johto. It is known as Johto's highest peak. It was featured in the special Mewtwo Returns. The mountain contains a lake with water containing healing properties known as Clarity Lake. Butterfree are also known to lay their eggs here. It is surrounded by Purity Canyon, Johto's greatest natural wonder, and Purity River, whose source is Clarity Lake. Because of its properties as well as it being a breeding ground, the Bug Pokémon view it as sacred, and will attack anyone who threatens its stability.
Pokémon researchers Luna Carson and Cullen Calix individually came to the mountain to research about the lake. In the special, Mewtwo and the Pokémon clones were said to have come there after the events of Mewtwo Strikes Back. In addition, storms were frequent in the vicinity, making it virtually impossible for people to access. (It is unknown why this is, although it is implied to be caused by Mewtwo in an attempt to ensure no one manages to stumble on them, although he won't go so far to allow a bus of civilians to die presumably for the same reason.) Giovanni, along with Domino and Team Rocket, came here to get back Mewtwo and exploit the healing waters of the lake, sparking the events of the special, but were later stopped by Ash and his friends. Mewtwo later wiped Team Rocket's memories and moved the lake to the inside of Mount Quena.