First, someone might pour molten turkey fat down a drain. A few blocks away, someone else might flush a wet wipe down a toilet. When the two meet in a dank sewer pipe, a baby fatberg is born.

Eventually, more fat, oil, and grease congeal onto the mess and build up into giant stinking globs. When they get big enough, fatbergs can clog sewers entirely, sending raw sewage gushing into streets.

On September 12, workers in London discovered one of the biggest fatbergs ever seen, in the East End neighborhood of Whitechapel. Measuring more than 800 feet long, the monstrosity weighs an estimated 130 metric tons and is the size of 11 double-decker buses, according to the London utility provider Thames Water.

800-Foot-Long Blob of Fat and Trash Found in London Sewer

The utility says the new fatberg is nearly 10 times larger than one pulled from the sewers of Kingston borough in 2013, which was found when many of the neighborhood’s toilets backed up.

Fatbergs are a sewer scourge, and both the nasty blobs and the fights against them have been growing. London, Belfast, Denver, and Melbourne are just a few of the world metropolises that have discovered large fatbergs in recent years.

View Images London sewer workers discovered this fatberg stretching at least a city block under Whitechapel Road. Photograph Courtesy of Thames Water

When the bus-size Kingston fatberg was discovered, a supervisor for Thames Water told the BBC: “We reckon it has to be the biggest in British history.” Within two years, an even bigger one snapped sewer pipes a meter wide in the London neighborhood of Chelsea. This summer, Northern Ireland Water excavated “a couple of hundred tonnes” of grease and debris from a fatberg underneath a row of fast-food restaurants in Belfast.

View Images Chunks of fatberg are surprisingly hard; the utility Thames Water compares removing them to breaking up chunks of concrete. Photograph Courtesy Thames Water

The problem isn’t just gross; it’s also a financial drain. In New York City, grease causes 71 percent of sewer backups, according to the city’s 2016 State of the Sewers report. The city spent $18 million over five years fighting fatbergs. Smaller cities aren’t immune; Ft. Wayne, Indiana, has spent half a million dollars a year cleaning grease out of sewers. (See a video of Ft. Wayne’s fatty pipes.)

The United States and United Kingdom report the most fatbergs, says engineer Thomas Wallace of University College Dublin, who studies the disposal of fat waste. Not only do both nations produce copious fatberg ingredients, but they also have many aging sewer systems ill-equipped to deal with the onslaught of fat and trash from growing populations.

Fighting the Fatbergs

Clogging is a problem as old as sewers themselves; the ancient Romans reportedly sent public slaves underground to clean their sewers. But the enormous fatbergs of today are brought on by more modern inventions.

The first fatbergs probably started small, as cities and their cooking waste grew with the industrial age. In 1884, Nathaniel Whiting of San Francisco patented the first grease trap to catch “substances which would tend to choke and clog the sewers.”

His basic design is still used today: Wastewater drains into a box where fat settles out. Eventually, someone has to clean the gunk out and dispose of it.

View Images A technician shovels fat out of the London sewers in 2014. Photograph by Adrian Dennis, AFP, Getty Images

In the United States, many cities eventually required restaurants and other food sellers to have grease traps and to clean them out, and a surprising amount of controversy and intrigue has grown around these caches of fat. In some places, thieves blowtorch their way into grease traps to steal used cooking oil that can be made into biofuels.

In China, fat from sewers and traps is illicitly scooped, cleaned up—though not well—and sold on the black market as “gutter oil.” In cheap restaurants and street stalls, your dinner might even be cooked in gutter oil.

In the U.K., grease-trap rules have been more lax than in the U.S., because many water systems such as London’s Thames Water are privately owned and don’t have much authority to enforce their use. Instead, the utility hires a team of “flushers,” people charged with digging out fat and other nasties to keep pipes flowing.

So it’s fitting that the word “fatberg” was coined by the people who know fatbergs best: the sewer workers of Thames Water. The description, conjuring up a pale floating mass of epic proportions, is far catchier than the American version—the acronym FOG, which stands for fat, oil, and grease. Fatbergs caught on well enough to make it into the Oxford English Dictionary in 2015, alongside “Brexit.”

Sewer Soap and Song

As fatbergs have grown, scientists have learned more about how they form and how to fight them. For starters, scientists only recently discovered that most of the mass in fatbergs is actually a form of sewer-made soap.

In 2011, Joel Ducoste of North Carolina State University and his team reported that the same process that can turn lard into soap, called saponification, happens in sewer grease if calcium’s around. The team even created miniature fatbergs in the laboratory that grew on calcium-rich concrete, a clue to how the blobs get so massive in certain sewers.

And in places where fatbergs are on the rise, sewer managers point to wet wipes as a major part of the problem. These pre-moistened toilet wipes are made for both babies and adults, and while many are sold as “flushable,” poorly dissolved wipes are pulled from sewers by the ton. Worse, the tough cloths can serve as excellent building blocks for fatbergs.

Now, Tom Curran of University College Dublin holds the first Fulbright scholarship awarded to a scientist in the fatberg fight. For his project, Curran will be working with Ducoste in North Carolina to map fatberg hotspots and develop sensors that would alert cities to ‘bergs before they reach pipe-bursting sizes.

Huge Blobs of Fat and Trash Are Filling the World’s Sewers This recent footage from a fatberg extraction in Belfast, Ireland, shows just how hard it is to remove the masses once they've congealed.

Some cities are even looking at fatbergs as legitimate fuel. After all, fat is high in calories and therefore energy. Thames Water has partnered with a renewable-fuel company to dig fatbergs out of sewers and turn them into biodiesel.

Curran says public awareness campaigns have already helped some cities reduce blockages by teaching people what not to flush or pour down drains. “There are also legal efforts in place regarding the use of the term ‘flushable,’” he says.