In one short email, Dan MacIntyre committed to history what could be the most romantic words ever: "Like wet wipes and congealed cooking oil, we are inseparable." When MacIntyre and his girlfriend Dunya Kalantery got together last year, they discovered a mutual interest in the giant "fatberg" – the UK's largest, formed of 15 tonnes of congealed cooking oil – that had just been found in the sewers beneath Kingston upon Thames. It took sewer workers three weeks to clear the huge lump, which was the size of a bus, with water jets.

"We were both fascinated by the story," says MacIntyre, 27, a learning support worker. What did they like about it? (As if I have to ask.) "Just the mixture of the different substances – the fat, the sanitary towels, and that as an image. It is really gross and we were fascinated in a disgusted way."

And so, to celebrate their first anniversary last week, MacIntyre decided he would like Kalantery to see a fatberg for herself – hence the email to Thames Water which, happily, was taking a BBC film crew down the sewer that night and allowed the couple to tag along ("We never take members of the public down our sewers, except for one week every May where we run a 'sewer week', so it was a rare opportunity," says a Thames Water spokesperson).

Dan MacIntyre and Dunya Kalantery enjoy their sewer bliss.

Kalantery, a 27-year-old art curator, loved the surprise. "I was extremely struck by it, and excited. We got to put on white protective suits, and waders up to our hips." They were each connected to a harness and disappeared down a manhole outside Selfridges shortly before midnight. "The sewers were impressive," she says. "The current of the water was really strong."

"It was also unexpectedly pleasant down there," says MacIntyre. "It wasn't that smelly and it was nice and warm."

Then, says Kalantery, "we saw the fat, which was pretty great." Sadly, there were no giant clumps, thanks to the maintenance by sewer workers, but there was enough for the grease enthusiasts to get excited about. It was congealed around the pipes – the result of cooking fat and oil being poured down drains that solidifies around items such as wet wipes and sanitary towels that have been flushed (they shouldn't be, but increasingly are), creating blockages. This causes sewage to back up, which can flood homes and streets.

Was the fat as fascinating as she had hoped? "Yes, completely." She touched it, she smelled it ("like solidified burnt oil"); she wanted to take some home as a souvenir, but it was impractical.

What will they do on their anniversary next year? "I don't know," says Kalantery, "but I'm going to have to come up with something even better."