H2O Powered Broadband.

Are the cables "food safe?"

I don't see why they can't be - you can make cables sheathed with the same basic material (PE) that many water pipes are made of after all.

The Americans have been using building air ducts to route data cables for generations - and have a special 'plenum grade' cable sheaths to suit (mostly to stop poisonous fumes be created during a fire that could then be carried to people's lungs throughout the building by the air duct), so the idea of slightly specialised cables for routing in human supply routes isn't new.

   - Andy.

I do recall about 20 years ago it being mooted that one could use the sewage system to route optical fibres. I do recall thinking that anyone working on it for repairs and additions would probably demand danger money

The only folk I know did anything like that for real were the 1980s London Pirate radio stations who would get their antenna feeds from roof to an empty upstairs flat via the soil stack, and would typically take the loo out to catch the cables on the way down.  They would also often plant the antenna mast in the top of the stack with a length  of  scaffold pole or similar.

Of course if the station was raided, and they left in a hurry and it all fell down the hole, it left a terrible problem for the building maintainence chaps and chapesses to sort out at the bottom..

No really new ideas.

Mike.

One could conspire to make "water hammer" to send slow speed info at the same time. Loosen a few screws and get Morse code option

That's not really a new idea either - see 1 min 15 seconds in to this clip from the 1955 film The Lady Killers. Note the use of the mallet to shake the airlock. Vaguely reminiscent of some plumbing when I was a kid, but even then I do not think it was quite that bad. (I'm not that old either )

The communication rather than power version is also alluded to in the far more recent film Paddington when the inmates in jail communicate by tapping on the pipes.

Mike.

There a number of fibre optic data cables installed in London sewers, generally restricted to the larger sewers that can be entered for installation work.

Some can be seen in this report https://www.28dayslater.co.uk/threads/fleet-cso-and-storm-relief-london-dec-16-feb-18.112167/

A couple of the pictures about halfway down the lengthy report show a group of four coloured plastic pipes, these are ducts for fibre optic cables.

Canal tow paths and tunnels make ideal routes and are well used.

A bit of a risk if a boater drives in a long mooring spike though.

Z.

I suspect a bigger challenge is finding and by-passing all the valves - there must be quite a few on an 11-mile run of pipe and I imagine that closing one would make short work of an optical fibre.

   - Andy.