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.
“Underground” is mentioned in the text above. I know that some people are considering to store heavy blocks connected to ropes in the shafts (?) of old coal mines that go down hundreds of meter. Use a single, very heavy block and you’re good to go. Though I don’t think that it holds a significant amount of energy (too lazy to calculate and no feeling for the possible dimensions of such a block). Also, digging new shafts should be prohibitively expensive for energy storage.
