Concerning Deane Yang's 4th point, permit me to cite the earlier MathOverflow question, "$C^1$ isometric embedding of flat torus into $\mathbb{R}^3$," which displays (and links to1) some amazing images from the Hévéa2 project's illustration of the Nash-Kuiper $C^1$ embedding theorem applied to the flat torus:
![corrugations][4]
1 Courtesy of Benoît Kloeckner.
2 The Institut Camille Jordan, the Laboratoire Jean Kuntzmann, and the Grenoble Gipsa-Lab.