I ventured down Brunel Tunnel the other week, as it was open to the public for the first time in years. Hailed as the 8th Wonder of the world, in Victorian times the tunnel (between Rotherhide and Wapping) allowed pedestrians to cross underneath the Thames.
In those days the tunnel would be lined with souvenir stalls. Those eager to boast of their experience would snap up as many as possible as this demonstrated how long they were able to stay underground. Brilliant!