Cutty Sark by crossing the Thames at a depth of 50 feet, taking the atmospheric Greenwich Foot Tunnel.
It's not as old as the nearby Brunel's Thames Tunnel, the world's first under a navigable river, but that one is currently being used by the East London Line.
However it is eerie and spooky enough: the echoing footfalls, the smell of damp and feeling the weight of water above your head.
It was just the thing to experience while re-reading the "Rivers of London" series 3rd book, "Whispers Under Ground" and by concentrating carefully I could almost sense the vestigia those books speak of.
There were more tunnels on the way home, taking the Docklands Light Railway and then the District line at Tower Hill: