Companies say it’s a better way to dispose of the Cold War-era material — and fix a shortage of nuclear fuel. But the plan has also faced criticism from nonproliferation experts.

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The dream of clean hydrogen has tantalized energy experts for years, but producing it has been tough. Many start-ups think the answer could lie beneath our feet.

A new wave of start-ups are trying to harvest emissions-free energy from inside the Earth, but the industry still faces significant challenges.

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