Greenwich Foot Tunnel was completed in 1902, and runs from Isle of Dogs on the North bank of the Thames to Greenwich on the South bank.
It’s 1200 feet long and only 50 feet deep – which doesn’t sound deep enough to me, you’d almost expect to see fish in there at that depth. The length of the tunnel is covered in white tiles, apart from a short stretch at the north end which is encased in steel as a repair to bomb damage in the Second World War. The tunnel is a Right of Way, so it officially has to be open at all times, and there is no charge for using it, so at any time day or night, you can walk underneath the Thames.
Personally I wouldn’t go down there at night, but that’s just me, I haven’t seen any reports of anything nasty going on down there, but a night time trip down there doesn’t sound sensible – especially being as it is underground, so looks exactly the same in broad daylight! Don’t be scared of a trip down there during the day though, there’s nothing scary, there’s a steady stream of people walking though, it seems a popular route for cyclists who were all dutifully obeying the request to walk with their bikes.
The tunnel joins two spots that have excellent views, the view from both sides to Canary Wharf is pretty spectacular, while from the North side there are good views across the river to the former Greenwich Hospital and Maritime Museum.
If you’re interested in a different view of London, I’d thoroughly recommend it.