It's a beautiful morning in Brighton: cloudless sky, crisp winter wind blowing in from the south and cresting surf crashing on to the pebble beach. I stand, hand in pockets, and take a deep breath of clean sea air. "What a lovely day," I say, to my friend. "It's almost a shame we're going underground."

"And into a shithole," she replies, screwing her mouth sideways.

I'm going down into the bowels of Brighton, journeying into its Victorian sewer system built, primarily, to keep the shit off the beach. I am haunted by visions of drifting poo and rivers of urine and I can't stop imagining that the smell is going to hit me like a freight train and make me never want to smell again. I'm also worried about touching anything. I don't want one speck of any stranger's effluent anywhere near me. "It is dirty down there," says Stuart, one of our guides. He's worked down here for more than 40 years. He's grinning from ear to ear. "Laurence Llewelyn-Bowen came here once. He wore a white suit. And he left without a single speck on him."

I'm wearing a white safety helmet and I've been given a numbered tag to wear around my neck "in case there's a flash flood and you're washed away to sea" explains Stuart.

I stare at him and blink. I'm in a holding room being shown a video about a man called Dr Russell who wrote a dissertation on the benefits of seawater for disorders of the glands. It's his fault they had to build a sewer. Between him and the Prince Regent, Brighton became a Must-See location and before they knew it, the town officials had more shit than they knew what to do with. And so the Victorians, who had sensitive noses and longed for clean pebbles, set about building the Brighton sewer system.

I'm yet to have a full blast of the smell. I have been assured that "once you're used to it" it's fine and that I should, "take a deep breath as you go in so that the senses at the top of your nose sort of die".

I am dreading this. My friend has already declared her intention to vomit and as we ready ourselves to go through the vault-like door and into the abyss, Robert, our other guide, turns round and quips: "It's a good job you're not here on a Saturday morning after all the beer and curry's been tipped into the system. Now that's a smell."

At this point I foolishly ask whether anyone has ever fallen in. Robert, to my horror, nods. "We have to get into it to clean the fat and remove the debris. I saw a man go under. He had to spit shit out of his mouth. Remember that. If you fall in, keep your mouth shut."

It is only now that I get my first experience of the smell. To be fair, it's not as bad as I thought it would be, and it is true that after the first sharp punch, you get used to it pretty quickly. All the same, it has a depth to it, slightly thick, cloying and with a sinister undercurrent of death and decay.

We walk through a narrow tunnel and come out into the "catch tank". Stuart shines a torch down into a flowing mass of sewage. "What do you think that is?" he asks. We look down and I can see, just under the flowing murky liquid, something unformed and anonymous.

I think it's fat. I've guessed that because Stuart has told us about the icebergs of fat that float through the system. The fat forms rafts so thick they can stand on them. But it's not fat. "That's sanitary towels," says Stuart, with a delightful smile. "Later, I'll show you where they really gather." He says this as though it's a massive treat.

"Once, when I was down there," he adds, "I found a severed finger."

I don't need to hear another thing. Stuart and Robert are the bravest men on earth. And don't put fat or sanitary towels or tampons down your toilets. Stuart asked me to tell you.

• Brighton's sewer tours (01903 272606, tinyurl.com/brightonsewers) run from May to September, and must be booked in advance. Adults £12, children £6 (minimum age for tours 11 years old)

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