Mr Smith goes to . . . The sewers

From down the drain, everything above ground looks good

Is the Augean gloop squelching and slurping under the sole of my wader what I think it is? Well, yes and no. To deal with "no" first: no, it's good old Thames mud, which you'd naturally find underfoot if you had just popped out of a sewer under the Chelsea Embankment, to emerge on the riverbed at low tide. Turning to the more nostril-wrinkling matter of "yes", I am here to join a crocodile of officials from the Environment Agency, who are worried that heavy rainfall tends to top up the Thames with raw sewage. Despite the best efforts of Thames Water, there are 40 effluent "overspills" per annum. If these were all laid end to end, in what accuracy compels me to call a time-and-motion study, it would be the equivalent of the sewers flooding into the Thames around the clock for more than 20 days a year.

Two hundred years ago, when that underrated city father, Sir Joseph Bazalgette, built the embankments and the intercept sewers, there would have been nothing remarkable about that. You get a sense of the brilliant simplicity of his design if you slither through the Ranelagh sewer, under the grounds of Chelsea Hospital. Near the river, the Ranelagh drops into the wider intercept culvert, which it meets at right angles beneath the embankment. The churning cataract of waste and water is swept away to treatment works in east London. In a real downpour, however, the Ranelagh will shoot the rapids of the intercept and burst through hatches in the riverbank.

Ben Nisdale of Thames Water is the closest thing we have today to a Bazalgette. He knows the sewers like the back of his trusty hygienic gauntlet. He's the man you want in front of you as you shuffle through the reeking gloom. The Environment Agency's preferred solution to the pollution problem is to erect giant screens across the sewer outflows. The unspoken assumption is that, by the time we've negotiated the odd rat carcass, and experienced the ghost-train thrill of dangling tree roots brushing our faces in the dark, we'll appreciate the practical difficulties.

We take a breather in the hospital gardens, where the strains of band practice carry from the nearby barracks. We're wearing hard hats with Davy lamps, and high-visibility jerkins over our boiler suits. When we move on to Pimlico, passers-by look at us in alarm. It's not until I see our reflection in a shop window that I understand. Dropping through a manhole cover, we might be looking for bombs. Nisdale is too preoccupied to notice. To paraphrase Wilde, we're all lying in the gutter, but some of us are looking at the sewers.