With its hulking solidity and its icy, aquamarine corridors reaching away into darkness, Blythe House can’t but call to mind the proverbial iceberg-below-the-waterline. Once the Victorian headquarters of the Post Office Savings Bank, the building’s recesses now hold some of the roughly 95 per cent of the major London museums’ collections not currently on public display. It’s a vast resource, and it is possible to visit, but access is hardly easy: aside from the logistical hassles of applying for permission and getting to or across London, even senior curators, once having navigated the building’s byzantine security, risk getting lost among their own skeletons every time they descend into its labyrinthine bowels.