The plan is still in the conceptual phase.
"The mine in question is the Mir mine in the Mirniy industrial zone in Eastern Siberia. You might have probably seen photos of it before, labelled "the largest man-made hole on earth" or something similar. It's long been used to mine diamonds, but the mining stopped in June 2001, replaced with a network of underground tunnels below. Presumably this activity would have to cease if the structure is ever built.
The project, which is described in a series of slides on AB Elise's site, is split between three levels, penetrated in the centre by a vertical farm and forests. Around the exterior are residences and recreational areas, and there's also a research centre in the plans."