It seems like a neat solution. The globe is pockmarked with disused mine shafts deep enough to house a full-sized Gravitricity installation, which will stretch down at least 300m (984ft), and possibly much further. There"s political will to make it happen too, Blair says, with policymakers keen to tap into public enthusiasm for a so-called "just transition" – the notion of a new, low-carbon economy that secures the livelihoods of fossil fuel workers and their communities. And so, with enough funding, a subterranean prototype (most likely located in the Czech Republic) should be functioning by 2024. First, though, a series of challenges must be overcome.