The Great White Throne of Zion! Wikipedia sys:

The term Great White Throne is taken from Revelation 20:11-15. Seventh Day Adventists believe that the Great White Throne in Revelation 20:11 is the Last Judgment when all people will stand in judgment before Jesus after their investigation judgment and a verdict of their salvation will be made.

According to the teaching of some Evangelical Christians — particularly among those Protestants who adhere to a millennialist eschatology — there will be two separate resurrections for the judgment of believers and non-believers respectively.

As for the mountain itself:

The Great White Throne was named by the Methodist minister of Ogden, Utah, Frederick Vining Fisher, in 1916.[2] On a trip up the canyon with Claud Hirschi, son of Rockville bishop David Hirschi, Fisher and Hirschi named many features in Zion Canyon. Later afternoon light gloriously lit up The Great White Throne, prompting Fischer to state:

“Never have I seen such a sight before. It is by all odds America's masterpiece. Boys, I have looked for this mountain all my life but I never expected to find it in this world. This mountain is the Great White Throne."