Cold fusion is a room temperature nuclear reaction concept which had incredible potential for humanity, but has been rejected by mainstream science. While there are variations (bubble and muon) of the theme, the effort that captured world attention was the announcement by electrochemists Stanley Pons and Martin Fleischmann on 23March1989 about their tabletop experiment. They claimed excess heat and production of neutrons and tritium. This result could not be reliably repeated. Reaction and condemnation were swift, as eight months later physicists nailed the coffin on what they called pathological science. I still think this dismissal was premature and the peta-system (relative to ITER, for example) remains hopeful for a breakthrough in the future. "Something happened," and in my mind, we don't yet know what.
A thought that occurred to me about how fusion might be stimulated at room temperature, and something that definitely should be explored, can be drawn from lightning bolts. Recent experiments have shown that thunderstorms can generate x-rays, gamma-rays, and, even, anti-matter. Astrophysicists once thought that only solar flares, black holes and supernovas could sufficiently accelerate electrons. Well, if something so simple as atmospheric electricity can do so much, it is just a matter of time before someone smart enough can repeatedly accomplish this in a simple laboratory. Benjamin Franklin, incidentally, might have first initiated this search.
Finally, here is a fusion process that works, my sunset tonight: