I still believe they will solve it some day!
Fusion.
When I was in college, about 50 years ago, I was taught that one barrel of ordinary water has energy content of 660 barrels of gasoline!
Burning water would be clean and incredible source of energy. And we would get Helium out of it, which we have shortage of right now.
But, 50 years later, we still have nothing!
Kind of depressing to thing about it.