> Actually, from what I've read, Einstein came up with the General Theory
> of Relativity completely out of the blue, based on no experimental
> evidence whatsoever. I find this to be the single most impressive
> mental feat of all time.
Previous to Relativity there had been experiments to show that the speed of light could be absolute and that time dilation could be necessary to explain some inconsistencies in electromagnetism. In fact Relativity was Einstein's answer to the apparent time dilation necessary in electromagnetism (there had been papers on the concept of relativity but apparently Einstein hadn't read them). Later Minkowski created the concept of spacetime. Einstein came up with the idea of spacetine curvature while investigating tidal gravity. Not exactly 'out of the blue' but still very impressive.
BM