Full Idea
Cantor's 1891 diagonal argument revealed there are infinitely many infinite powers. Indeed, it showed more: it shows that given any set there is another of greater power. Hence there is an infinite power strictly greater than that of the set of the reals.
Gist of Idea
It was Cantor's diagonal argument which revealed infinities greater than that of the real numbers
Source
report of George Cantor (works [1880]) by Shaughan Lavine - Understanding the Infinite III.2
Book Reference
Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.48