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The problem then being: how do you remove the mass you're extracting? Existing horizontal TBM systems make this much easier.

Don't, just price it into the project and leave it down there.

I meant the dirt, not the TBM itself.

Hmm yes, quite. Never thought about that haha.

Better idea (maybe): store even more gravitational energy by sending heavy weights into orbit. Just use booster rockets to make it fall through the atmosphere, regenerating vast amounts of energy in the process.

Geothermal looks promising, using the technology for drilling wells.