Many a frustrated physicist has complained that Mother Nature is a bitch. After the instability problems we described in previous chapters, fusion physicists would have agreed had the problems not been so challenging but soluble. There have even been several pleasant surprises when unexpected benefits were found that could not have been foreseen when fusion reactors were first envisioned. Some of these effects are now well documented; others still cannot be explained. The most remarkable of these surprises is the H-mode, a high-confinement mode on which present designs depend. It is so important that it deserves its own section, which follows this one at the next major heading.