Oh yeah, they're everywhere, we've been digging for millennia. Though as anyone who's bought a home around old mining areas knows there's a fair bit of subsiden

Oh yeah, they're everywhere, we've been digging for millennia. Though as anyone who's bought a home around old mining areas knows there's a fair bit of subsidence around old mines and shafts. Leaves me wondering what proportion is feasible without significant effort digging them out, pumping and propping them again? Then there's the countless seams that were narrow enough to trigger instant claustrophobia. The many crawlable only spaces that don't give space to sit, let alone stand. Plenty of headroom for lettuce, but rather more challenging for harvesting and the associated humans.