In some senses the whole planet is storing a huge amount of energy underground.

But yeah I think in the context of this kind of discussion it’s would be a renewable energy source. While it’s probably technically not renewable, but there’s so much of it we’d never run out. I mean if you want to be super-pedantic, solar and wind aren’t renewable either because we don’t have a way to make a new sun when that thing burns out, but it’s like come on.

But probably not a storage solution, because why would you put energy into the Earth’s core? We worried about hell freezing over or something?