It's gross beyond imagination. It's the fatberg

Picture a man like a grizzled Bruce Willis, or, if you prefer, a grizzled Sam Jackson, sitting in a comfortable chair in his modest home.

He wears threadbare slippers and a ratty bathrobe even during the day and he occupies himself with soothing hobbies that require quiet absorption - cryptic crosswords, say, or making model airplanes from balsawood, or needlepoint replicas of Monet paintings, the bright sunshiney open-air summertime ones. The hobbies help with the nightmares.

This man has seen things that you and I have not seen. He has looked beneath the skin of the world and grappled with the monsters that live there. Once before he fought the evil and prevailed, but he can't rest easy, for he knows that we make the monsters, so as long as we exist, they will return.

Even now, if he allows himself to close his eyes he can hear it squelching closer, seeking him out, blind, remorseless, a vast and voracious worm tunnelling through the dying heart of the world. He is not retired, not really. How can he be when at any moment the phone might ring and a low, urgent, official voice, tight with fear, will say: "We need you. It's back."

This time the terror strikes in Whitechapel. One hundred and thirty years ago Jack the Ripper stalked those streets and side-alleys but now the horror is underground. At 250m long, weighing 143t, the equivalent of 11 double-decker buses or 20 African bull elephants, the Whitechapel Fatberg discovered earlier this month is the biggest fatberg the world has seen so far, bigger than the Melbourne Fatberg of 2014, or the Belfast Fatburg of 2015, bigger even than the fatbergs in Chelsea and Shepherd's Bush in 2016. London is the fatberg capital of the world but they're spreading, and they're getting bigger, smarter, more adaptive.

What is a fatberg, you ask? Is it the original surname of my dentist Morris Finkelberg before he wisely had it changed? Is it the technical term for that moment in middle age towards which all you skinny millennials on your blithe Titanic of youth are barrelling, full steam ahead?

No. A fatberg is a vast accretion of grease and cooking fats, cohering and hardening like a viscid tumour around the binding material of semi-solid flushed items and sanitary wear. It grows in the pipes and sewers beneath our feet, revealing itself first in small signs and oleaginous auguries all too easily overlooked: rats come from the sewers as though chased; a cat wanders down a drain and never comes back; a small child is found with staring eyes, encased to the waist in gunge, unable to speak. The first the rest of us know is when our toilets stop flushing.

Next, sewage and human waste erupt into our streets and our homes in high-pressure jets and geysers, in great fountains of effluvia, the return of a monstrous repressed. We want to keep ignoring the consequences of our gross lives, but we cannot. The fatberg is here! Call the fatberg wrangler! And then Bruce or Sam must don headlamps and overalls and kiss the photos of their family goodbye, and go forth once more to confront our mucilaginous id.

There is something repulsively human about this creature that we create with our own excess. The principal solid ingredients of a fatberg are intimate wipes, used tampons, used condoms and soiled nappies - entire life-cycles of human beings. Whole relationships will be preserved in there, like loathsome flies in unwholesome amber. You could send forensic investigators armed with DNA kits tunnelling into a fatberg and reconstruct your courtship, marriage, first child and divorce, all preserved and suspended in a gelatinous time capsule.

The fatberg is human in this way too: it's the subterranean consequences of our lives made visible. All the trash we discard goes somewhere; all the emotions we repress and questions we avoid and decisions we outsource go somewhere; all the ripples and repercussions of our words and actions and omissions ripple out and fetch up somewhere, unseen and for someone else to deal with.

The fatberg exists to remind us that out of sight may be out of mind but it isn't gone forever, and there will come a time when Bruce or Sam aren't there to do our dirty work for us any more. One day the fatberg will come to all of our doors and rap with its congealed knuckles. Ask not for whom the fatberg knocks. It knocks for thee.