New Newgate Prison

They say that where the stalactite meets the cavern roof, they've laid charges of blasting powder, and if the prisoners ever riot, they'll blow the charges and let the whole thing drop into the Unterzee. It'd make quite a splash.

London's major prison post-Fall is a fortress carved from a massive stalagtite. All prisoners are required to wear masks.

Supplies and prisoners are brought in and out of the prison via airship. Strangely enough, these supplies include large quantities of candles. 

In the tip of the stalagtite are the cells for those in solitary confinement, and is also where one could find the oubilettes. Some prisoners are said to have heard frightful whispering from that vicinity.

Unpleasantly, prisoners have been known to be found with their masks missing, with swathes of the skin beneath torn away.