Reached via glass-topped domes (with lifts and steps) on either side of the river, this white-tiled Victorian-era 370m-long pedestrian tunnel provides an atmospheric back route from Greenwich to the Isle of Dogs; it's also an excellent diversion on foot to get a photo-op of Greenwich from the north side of the river. At an average walking pace, the journey is only around 10 minutes, but can be done faster – especially if you aren't keen on the feeling of the river just 50ft overhead!
Note the regulations: 'No cycling, busking, animal fouling, littering, loitering, skateboarding, skating, spitting. Dogs to be kept on leads. Keep left'. About 1.2 million crossings are made by pedestrians every year. There is another foot tunnel under the River Thames at Woolwich.