A wreck-diver buys a dying guildmaster's map to a Waterloo strongroom that opens four hours a year, at the lowest tide, and seals for another decade after.
As of2066
WhereFlooded London, Waterloo Interchange Relay, The Waterloo strongroom, Tide Roads
Last master of the Ferris Guild. Dying of the thing the Tube gives you, more honest than usual, and holding the map to a strongroom under the Waterloo Interchange.