I feel this idea would mesh well with the mining industry. Dig straight down, reinforce the walls on the way, separate out the desired minerals, pull up the drill, put in a pendulum and move the rig over and start again. You already need to setup giant motors to move the pendulum, so getting the ore up would be easy. An ideal site would have an abundance of lead, which would cut down on the pendulum cost. Unused rock could be mixed into cement for the walls. The small footprint combined with the benefits of being able to store solar or wind energy for long periods of time could make it the first form of mining that environmentalists approve of.