"Trash It. Don’t Flush It." ads have been popping up all around New York City. You may have seen them in subway stations, on the train, in bus shelters, or even on social media.

The ads are all part of a campaign the Department of Environmental Protection started to warn residents about so-called "fatbergs" — icebergs of sorts made of fat, not ice.

When grease is poured down the sink, it combines with items that can clog the sewers, sticking together and forming fatbergs, DEP says. 

The city spends $19 million a year transporting unflushable items to landfills.

DEP wants New Yorkers know that fatbergs are a growing issue — and that there are ways to prevent them. 

Fatbergs aren't only an issue in New York City. In London, a 210-foot-long fatberg was found blocking a sewer, according to DEP.