10E6 shafts is not a scary number. There are about 10E7 active oil wells in the US. https://www.eia.gov/petroleum/wells/ https://www.eia.gov/petroleum/wells/

10E6 shafts is not a scary number. There are about 10E7 active oil wells in the US. https://www.eia.gov/petroleum/wells/ https://www.eia.gov/petroleum/wells/ There are 10E8 private water wells in the US. https://www.epa.gov/privatewells https://www.epa.gov/privatewells So 10E6 sounds practicable and, as discussed previously, may be a significant overestimate of actual need. Suspect 50 year life cycle costs and various environmental and political externalities would be substantially lower for gravity-based solutions. But who knows? Concrete requirements? About 10E10 tons of concrete are consumed worldwide each year. https://concretehelper.com/concrete-facts/ https://concretehelper.com/concrete-facts/ Again, 10E6 doesn’t look so big, but might involve some engineering cleverness. But 10E6 big, capital intensive, fairly generic things can justify a lot of engineering cleverness in that one generic design. Geotech would need a lot of thought.