I have been following them for a few years and they are making some slow and steady progress
From their page: As the world generates more electricity from intermittent renewable energy sources, there is a growing need for technologies which can capture and store energy during periods of low demand and release it rapidly when required.
At Gravitricity we are developing innovative, long-life, underground technologies which store energy safely and deliver it on demand at a lower lifetime cost than current alternatives.
I once saw images of 100’s meters deep, ~10 meter wide ventilation holes of african mines (of course unsecured). So there are “drills” for that. Why not use something like this for hydro power storage, without the hull and all? Sure, the ground needs to be suitable.