Yes but the only places we care about extracting heat are the tunnels, so you'd just run pipes through the tunnels themselves to extract heat. I think the bigge

Yes but the only places we care about extracting heat are the tunnels, so you'd just run pipes through the tunnels themselves to extract heat. I think the bigger issue is the amount of energy needed to get the water down there and back up just means it doesn't make much sense on its own.