The Rotherhithe tunnel is an old hole under the Thames river, nearly a mile long, intended to accomodate both vehicular and pedestrian traffic. In the 112 years since it opened, much has changed about the nature of vehicles, and it is now reportedly "inadvisable" for pedestrians to enter. Once walked by 14,000 people a day, Wikipedia now claims only about 20 people a day make the trip.
This man is among them, though, and he and filmed it for our mild horror.
EXHAUST FUMES. DO NOT LINGER.