Imagine doing these while improvising... (transcribed by me)

January 7, 1965: Carnegie Hall, New York City, New York (Studio)
Horowitz was preparing for his momentous return in 1965, after a hiatus of twelve years. Given the profound influence he had previously left on the music world, this was an event that was bound to create massive ripples.
In preparation for his once again debut at Carnegie Hall, he went through multiple rehearsals by renting the entire Carnegie Hall without an audience (which in itself is an extraordinary undertaking). This is an improvisation that he played to test the piano and warm up his hands before all practices during his first rehearsal in January 1965. Damn it, he must have either composed it beforehand or otherwise, there can be no doubt that he is mad (either way, it's epic). Just think about being able to play and utilize such passages routinely. That is why he is called a maestro...