The overtaxing of aging sewage systems and the popularity of sanitary wipes are creating a new environmental and public-health problem in the world’s cities known as “fatbergs.”

Fatbergs are colossal masses of sanitary wipes (of both flushable and non-flushable variety), congealed fat, feces, and a variety of materials that are clogging up sewer systems in Great Britain, Australia, and now here in the U.S. Some fatbergs are reportedly as big as jumbo jets and as hard as concrete.

It's believed that fatbergs grow quickly, and can cause sewage to back up on streets and lawns through manholes, and even flood homes through their toilets and sinks. Besides causing tremendous sewage backups, they can destroy aging waste water systems and equipment, and can take months and costs millions to remove.

Sewage departments in Great Britain caution that fatbergs are becoming more common, especially in cities where great amounts of cooking oils are incorrectly poured down drains. The sanitary wipes, which also make their way into the sewage systems, trap the congealed oil and sticky messes build up on sewer roofs like mushy stalactites. These masses eventually trap other sewer matter like feces and garbage and create a clot in the system. Sometimes a fatberg may break off from a sewer wall, move through the sewage system, and collide and join with another fatberg.

"I have witnessed one,” a water company spokesperson told the UK’s Guardian newspaper. “It's a heaving, sick-smelling, rotting mass of filth and feces. It hits the back of your throat, it's gross.”

As London has huge sewers, their fatbergs can become really immense. The Thames Water company, which operates in London, clears nearly 55,000 blockages a year caused by fat and sanitary wipes. Often, workers use industrial vacuums and high-pressure water hoses to suck away or break through the congealed fat, but sometimes the rock-hard masses have to be removed manually. One London fatberg caused extensive damage to the sewers in Kingston, taking more than six weeks to repair.

While some fatbergs are relatively small (many are described as being about the size of backpack), there have been some notably massive fatbergs found in recent months, including:

A fatberg the size of a London double-decker bus that clogged up the drains under London Road in Kingston upon Thames on in August, 2013.

A fatberg the size of a Boeing 747, which also contained wood planks and tennis balls, was found in West London in September.

A gigantic fatberg coated the walls and clogged the sewage system beneath Melbourne, Australia in September. It was described as being hard as a rock and took workers four weeks to remove.

Fatbergs are also beginning to have a negative impact on sewage systems here in the U.S. In September, a fatberg crippled part of the sewage system in Putnam County, West Virginia. Sewage workers there discovered clusters of disposable wipes that clogged a piece of equipment that’s meant to grind down sewage into fine particles. But since sanitary wipes are not water soluble, they didn't dissolve and jammed up the machine and burned out its motors.

And it’s not just West Virginia. Flushable and non-flushable sanitary wipes are starting to cause problems in New York and New Jersey. New York City spends some $18 million a year to remove debris that gets caught in its pipes and the machinery at its wastewater facilities. And according to the city’s Department of Environmental Protection, it’s nearly all flushable wipes.

“The increase in clogs and problems we’ve been having in New York City," the DEP’s Commissioner told the New York Post, “seems to almost correlate directly with the increase in sales of these flushable wipes.”

The Post reports that New York’s DEP now carts away some 110,000 cubic yards of debris each month from its treatment plants, which is double the volume of just five years ago.

Watch this video of a huge fatberg in a London sewage system: