Nuclear fusion is the ephemeral holy grail of climate technology. It would provide nearly limitless amounts of clean energy without the byproduct of long-lasting radioactive waste to be managed.

It’s also the biggest bet Silicon Valley luminary Sam Altman has ever made.

“This is the biggest investment I’ve ever made,” Altman told CNBC of his $375 million investment in Helion Energy, announced Friday. It’s part of a larger $500 million round that the start-up will use to complete the construction of a fusion facility near its headquarters in Everett, Washington.