At some point over the past couple of months, a fry cook in Belfast poured some extra grease down the drain. Then another cook, pressed for time, emptied her own tray into the sink. Around the same time, a harried parent disobeyed a bathroom sign and flushed some baby wipes.

Unseen, below the streets, these small decisions built up. Hot grease cooled in the pipes and congealed. Baby wipes and tampons stuck to the growing mass. And a gross, smelly villain slowly took shape: a fatberg, the scourge of sewers everywhere.

The term “fatberg”—a portmanteau of “fat” and “iceberg”—is used by authorities to describe the big globs of refuse that build up inside sewers when people flush things they shouldn’t.

They have the potential to get very big and very gross. In August of 2013, a record-setting fatberg wrestled out of a sewer in London ended up being the size of a bus. A year later, that record was broken by another London ‘berg, this one the size of a Boeing 747.

As the BBC reports, this particular fatberg has made its home in a sewer beneath Dublin Road, and had nearly broken free onto the street by the time it was discovered. Crews from Northern Ireland Water have worked to remove it for six straight Sundays, and have already pried out “a couple of hundred tonnes” of grease, the outlet says.

The discovery inspired NI Water to release a kind of sewer-system audit: a list of everything weird they’ve found down there in the past. It’s a good read, and reveals that at least one fatberg was providing food for “a family of frogs.”

It will likely take a few more weeks to get the fatberg out completely. And then—if people don’t change their habits—the slow drip of grease and sanitary products may start the entire process over again.

Every day, we track down a fleeting wonder—something amazing that’s only happening right now. Have a tip for us? Tell us about it! Send your temporary miracles to [email protected].