The statue of James Greathead (concealing a ventilation shaft for the Northern Line at Bank) whose tunnelling shield made the building of the City & South London Railway a possibility. He didn't invent the tunnelling shield. Marc Brunel did. Greathead's shield was circular so it could support a greater weight of earth than Brunel's square shield. Deep level tunnelling was far less disruptive than the earlier cut & cover method used for the Metropolitan Railway.