Banish all thoughts of nauseating fetid stenches and unhygienic unmentionables. For when you descend into the bowels of Paris to visit its sewers you will find that the air is merely a little stale, and the only thing visible in the fast flowing streams of grey water are some autumnal leaves. No need for waders either, as you can watch everything dry-shod from the safety of walkways and bridges above these subterranean canals.
You have to hand it to the Parisians; they do things with panache. Not for them a discreet and hidden sewerage system. No, theirs must be proclaimed the best in the world and opened to the public as a tourist attraction. Only Paris could get away with it.
Amazing engineering
The French call them les egouts - so much more melodious than "sewers". There is even a rallying cry, in true egalitarian, Gallic fashion - "Tout a les egouts!" ("Everything to the sewer") - from the days when citizens were exhorted to send all their liquid waste to the (then) new disposal system.
That disposal system is a truly amazing work of 19thcentury engineering, built when people were just beginning to realise the importance of public health systems, of clean drinking water and covered sewers. So if you are at all curious about how cities function, the egouts are well worth le detour.
They lie beneath Paris like a second - a subterranean one where, in place of grand avenues and boulevards, you have broad, fast-flowing canals. And in place of small streets and alleys are smaller channels and rivulets.
At each junction the street names are set into the high, vaulted wall, just as they are above ground, so that you can easily orient yourself, despite a lack of obvious landmarks. Here the Quai d'Orsay, there rue Bougainville.
Even the smallest channels are wide enough for a person to work in, while the main, "arterial" sewers are big enough for boats. Yes, boats!
The novelist Victor Hugo called this vast network "the city's conscience", and he used it for a memorable escape scene in his novel Les Miserables.
The network was conceived in 1850 by the prefet of Paris, Baron Haussmann, and the engineer Eugene Belgrand. Above ground, Haussmann was remodelling Paris, introducing broad streets and a planned grid, primarily to facilitate easy troop movements and to control crowds, memories of the revolutionary barricades being still fresh.
Drinking-water pipes
Below ground, Belgrand produced an equally well-designed network, but one that is more than simply a sewerage system. For while the Paris sewers carried away the city's waste water, they also held the city's drinking-water pipes, and the canal-water supply that Parisians still use in place of sweepers to keep their streets clean. There are spare channels, too, to absorb flooding during storms; and a clever system of valves to regulate the level of the River Seine and prevent the sewers from overflowing.
As well as this comprehensive network and supply system, Belgrand and his team designed various tools and techniques to keep the sewers running freely. "Flushing boats", for example, are equipped with a large shovel-like board at their prow; as they chug their way down the main sewers, these boats clear the sediment that accumulates at the bottom of the channels.
Another of Belgrand's novel ideas was to take the waste away from the city to sewage fields where the effluent could be spread out and disposed of (after the first World War a modern treatment plant replaced the sewage fields).
Most of the water in the network falls under gravity, but in low-lying areas pump houses were added to lift the water downstream, push it under the Seine and north away from Paris.
By 1878 there were 600 kilometres of channel; today there are over 2,000 kilometres. Overhead, along the ceiling, and together with the pipes conveying drinking water and non-potable water, there are now telephone cables and other services.
Dublin has just woken up to the merits of placing such services in a sewer: 150 years after Haussmann and Belgrand, a US company, CityNet Telecom, was reported in The Irish Times recently as planning to run fibre optic cable through Dublin's sewers.
Exhibition area
The public tours of the Paris sewers visit a short section by the Quai d'Orsay where there there is an exhibition area, "Your Sewer Museum", showing the historic development of Paris and its sewer system over the past millennium. Boiler-suited sewer workers, justly proud of their role in keeping the city clean and functioning, will answer any questions you might have.
Back above ground, you will find yourself paying more than usual attention to manhole covers, and becoming acutely aware of the kerbside slits and drains that convey the street water to the sewer channels below. And you will flush your loo a little more thoughtfully, too!