Metamath is marvelous. Its proof explorer [1] has some 18,000 theorems (and counting) that are ultimately derived (and proved) from a small set of axioms [2]. Just looking at the list of these theorems makes me dizzy.
[1] http://us.metamath.org/mpegif/mmtheorems.html
[2] http://us.metamath.org/mpegif/mmset.html#axioms
